Light-emitting semiconductor device, light-emitting system and method for fabricating light-emitting semiconductor device

ABSTRACT

A chip-type light-emitting semiconductor device includes: a substrate  4 ; a blue LED  1  mounted on the substrate  4 ; and a luminescent layer  3  made of a mixture of yellow/yellowish phosphor particles  2  and a base material  13  (translucent resin). The yellow/yellowish phosphor particles  2  is a silicate phosphor which absorbs blue light emitted by the blue LED  1  to emit a fluorescence having a main emission peak in the wavelength range from 550 nm to 600 nm, inclusive, and which contains, as a main component, a compound expressed by the chemical formula: (Sr 1−a1−b1−x Ba a1 Ca b1 Eu x ) 2 SiO 4  (0≦a1≦0.3, 0≦b1≦0.8 and 0&lt;x&lt;1). The silicate phosphor particles disperse substantially evenly in the resin easily. As a result, excellent white light is obtained.

TECHNICAL FIELD

The present invention relates to light-emitting semiconductor devicesutilizing blue-light-emitting diodes (hereinafter, referred to as blueLEDs) and yellow/yellowish phosphors in combination to emit white light,light-emitting systems using the light-emitting semiconductor devices,and methods for fabricating the light-emitting semiconductor devices.

BACKGROUND ART

A light-emitting semiconductor device utilizing a blue LED (strictlyspeaking, a blue LED chip) having a main emission peak in the bluewavelength range from 400 nm to 530 nm, both inclusive, and aluminescent layer containing an inorganic phosphor (hereinafter, simplyreferred to as a “phosphor”) that absorbs blue light emitted by the blueLED and produces a fluorescence having an emission peak within a visiblewavelength range from green to yellow (in the range of about 530 nm toabout 580 nm) in combination is known to date. Light emitted from an LEDthat excites a phosphor is herein referred to as “excitation light”. Thespectrum of the light is herein referred to as an “excitation spectrum”.The intensity peak thereof is herein referred to as an “excitation lightpeak”.

Such a light-emitting semiconductor device is disclosed in JapanesePatent No. 2927279, Japanese Laid-Open Publication Nos. 10-163535,2000-208822 and 2000-244021, for example.

In Japanese Patent No. 2927279, a light-emitting semiconductor deviceutilizing a blue LED using a gallium nitride-based compoundsemiconductor as a light-emitting layer and having an emission peak inthe wavelength range from 400 nm to 530 nm, both inclusive, and an(RE_(1−x)Sm_(x))₃(Al_(y)Ga_(1−y))₅O₁₂:Ce phosphor (where 0≦x<1, 0≦y≦1and RE is at least one rare-earth element selected from among Y and Gd)(hereinafter, referred to as a “YAG-based phosphor”) in combination.

Considering the fact that the YAG-based phosphor produces an emissionhighly efficiently at a peak around 580 nm (yellow light) under bluelight emitted by the blue LED (excitation light), it is described in thepatent that the light-emitting semiconductor device is implemented as awhite-light-emitting semiconductor device which emits white light byadding the colors of the blue light emitted by the blue LED and of thelight emitted by the YAG-based phosphor together.

In Japanese Laid-Open Publication No. 10-163535, disclosed is awhite-light-emitting semiconductor device utilizing a blue or violet LEDand one or more types of phosphors each absorbing light emitted by theLED to produce emission in a visible range in combination. As aphosphor, blue, green, yellow, orange and red phosphors containing (Zn,Cd)S as a phosphor base and a (Y, Gd)₃(Al, Ga)₅O₁₂:Ce, Eu phosphor aredisclosed. The (Y, Gd)₃(Al, Ga)₅O₁₂:Ce, Eu phosphor is considered aYAG-based phosphor from a scientific standpoint.

In addition, in Japanese Laid-Open Publication No. 10-163535, alsodisclosed is a white-light-emitting semiconductor device producing anemission by adding the color of lights from the blue LED to the color ofthe YAG-based phosphor in which an emission chromatically point (x, y)of the emission is in the range 0.21≦x≦0.48 and 0.19≦y≦0.45 in a CIEchromaticity diagram.

Further, in Japanese Laid-Open Publications Nos. 2000-208822 and2000-244021, white-light-emitting semiconductor devices utilizing blueLEDs and YAG-based phosphors in combination are disclosed. In JapaneseLaid-Open Publication No. 2000-244021, disclosed is a light-emittingsemiconductor device utilizing a strontium sulfide red phosphoractivated by europium (SrS:Eu), in addition to a YAG-based phosphor, soas to compensate for a shortage of luminous flux of white light in ared-wavelength range emitted by the white-light-emitting semiconductordevice.

It is known that such a known YAG-based phosphor has a main emissionpeak wavelength that varies in the range of about 530 nm to about 590 nmdepending on the composition, especially the amount of Gd (gadolinium)atoms substituting Y (yttrium) atoms constituting the YAG-basedphosphor, the amount of addition of Ce³⁺ to be a luminescent center, oran ambient temperature. It is also known that the emission peakwavelength shifts to longer wavelengths as the substitution amount ofGd, the amount of addition of Ce³⁺ to be a luminescent center or theambient temperature increases. (see, for example, “Phosphor Handbook”:Ohmsha, Ltd. or a literature: R. Mach et and G. O. Mueller: Proceedingsof SPIE Vol. 3938 (2000) pp. 30–41). It should be noted that a Gd atomis heavier than an Y atom, and therefore the absolute specific gravityof the YAG-based phosphor increases as the substitution amount of Gdatoms increases.

It is known that the absolute specific gravity of a Y₃Al₅O₁₂:Ce³⁺phosphor containing no Gd atoms (in which the amount of Ce substitutingY is 0.1 to 2%) is 4.15 to 4.55 and that the peak emission wavelength ofthe phosphor at room temperature is around the wavelength range from 530nm (if the phosphor has an absolute specific gravity of 4.15) to 557 nm(if the phosphor has an absolute specific gravity of 4.55), i.e., thewavelength range from green to yellow/yellowish (excerpts from PhosphorIndex (Nichia Kagaku Kogyo Kabushiki Kaisha) and a catalog of PhilipsCorporation).

Now, color control of light, especially of white or whitish light,emitted by a light-emitting semiconductor device is briefly described.Conventionally, the color of light is controlled mainly by the followingthree methods.

(1) A method for obtaining a desired color of light by changing theoutput ratio between blue light emitted by a blue LED andyellow/yellowish light emitted by a YAG-based phosphor

(2) A method for obtaining a desired color of light by changing thecolor tone of blue light emitted by the blue LED

(3) A method for obtaining a desired color of light by changing thecomposition of the phosphor or the amount of addition of Ce³⁺luminescent center and changing the color tone of yellow/yellowish lightemitted by the YAG-based phosphor

Almost all the known light-emitting semiconductor devices utilizing blueLEDs and phosphors in combination as described above so as to obtaincolor-mixed light of the emissions from the blue LEDs and the phosphorsuses YAG-based phosphors as phosphors.

In the patent and laid-open publications described above, described are:a light-emitting semiconductor having a structure in which a blue LED ismounted in a cup provided in a mount lead and is electrically connectedthereto and in which a resin luminescent layer including a YAG-basedphosphor is provided in the cup; a light-emitting semiconductor devicehaving a structure in which a blue LED is placed in a casing and a resinluminescent layer including a YAG-based phosphor is provided in thecasing; a light-emitting semiconductor device having a structure inwhich a flip-chip-type blue LED is mounted on a submount element and iselectrically connected thereto and in which the flip-chip-type blue LEDis molded with a resin package also serving as a luminescent layerincluding a YAG-based phosphor; and like devices.

Such light-emitting semiconductor devices are known as light-emittingsemiconductor devices which are capable of obtaining white light andtherefore are in high demand for light-emitting systems such asillumination systems or display systems.

On the other hand, some of the light-emitting semiconductor devicesutilizing inorganic compounds other than YAG-based phosphors and LEDs incombination are previously known. For example, in Japanese Laid-OpenPublication No. 2001-143869, described is a light-emitting semiconductordevice using a silicate phosphor such as a Ba₂SiO₄:Eu²⁺ phosphor, aSr₂SiO₄:Eu²⁺ phosphor, a Mg₂SiO₄:Eu²⁺ phosphor, a (BaSr)₂SiO₄:Eu²⁺phosphor, or a (BaMg)₂SiO₄:Eu²⁺ phosphor.

In addition, in the same Japanese Laid-Open Publication No. 2001-143869,the wavelength range of light emitted by an LED is preferably 430 nm orless, and more preferably in the range of 400 nm to 430 nm. In anembodiment of this publication, a light-emitting semiconductor deviceusing an LED that emits light in the wavelength range of 343 to 405 nmis described. Further, the publication describes applications of thesilicate phosphors as green phosphors and also describes that it is morepreferable to use an organic LED than to use an inorganic LED made of aninorganic compound in terms of luminous efficacy.

That is to say, the invention disclosed in Japanese Laid-OpenPublication No. 2001-143869 relates to a light-emitting semiconductordevice utilizing an LED emitting near-ultraviolet light and a phosphorof an inorganic compound emitting red, green or blue light incombination.

Now, a silicate phosphor is described. A silicate phosphor expressed bythe chemical formula (Sr_(1−a3−b3−x)Ba_(a3)Ca_(b3)Eu_(x))₂SiO₄ (wherea3, b3 and x are in the ranges 0≦a3≦1, 0≦b3≦1 and 0<x<1, respectively)is known to date. The silicate phosphor, which was studied as a phosphorfor use in a fluorescent lamp, is known as a phosphor that emits lightwhose peak wavelength varies in the range from 505 nm to 598 nm, bothinclusive, by changing the composition of Ba—Sr—Ca. In addition, thesilicate phosphor is disclosed as a phosphor exhibiting relativelyhighly efficient emission of light when irradiated with light within therange of 170 to 350 nm in a literature (J. Electrochemical Soc. Vol.115, No. 11(1968) pp. 1181–1184) and a literature (Fluorescent LampPhosphors, Kith H. Butler, The Pennsylvania State University Press(1980) pp. 270–279), for example.

However, in the literatures about the silicate phosphor, it is notdescribed at all that the silicate phosphor exhibits highly efficientemission of light even in a wavelength range more than 350 nm,especially in the blue wavelength range greater than 430 nm and lessthan or equal to 500 nm. Thus, it is not previously known that thesilicate phosphor can function as a phosphor that emits light in theyellow-green to orange wavelength range from 550 nm to 600 nm, bothinclusive, especially yellow light, as a YAG-based phosphor, whenexcited by blue light in the blue wavelength range described above,especially blue light with high color purity around the wavelength rangeof 450 to 470 nm.

Hereinafter, the light-emitting semiconductor devices utilizing blueLEDs and YAG-based phosphors in combination will be described again. InJapanese Patent No. 2927279, Japanese Laid-Open Publications Nos.10-163535, 2000-208822 and 2000-244021 mentioned above, for example, thethickness of a luminescent layer in a light-emitting semiconductordevice and a fabrication method of the device are disclosed.

For example, in Japanese Patent No. 2927279 and other publications filedby the same applicant, a technique (pouring technique) for pouring anepoxy resin which is used as a base material for a luminescent layer andincludes a YAG-based phosphor mixed and dispersed therein into a cupprovided in a mount lead on which an LED chip is mounted, or into aspace of a resin casing and for curing the epoxy resin is used to form acoating containing the YAG-based phosphor on the LED chip. In thesepublications, the thickness of the coating containing the YAG-basedphosphor is set in the range of 100 to 400 μm.

In Japanese Laid-Open publication No. 2000-208822 and other publicationsfiled by the same applicant, disclosed is a technique for applying aphosphor paste made by mixing and dispersing a YAG-based phosphor in anepoxy region to the surrounding other than a mounting surface for an LEDchip and for curing the paste so that a luminescent layer is formed as apackage for covering an LED. In these publications, the thickness of apackage containing the YAG-based phosphor, i.e., a luminescent layer, isset in the range of 20 to 110 μm. In this case, a photolithographyprocess, a screen-printing process or a transfer process is used as amethod for applying the phosphor paste to the surrounding other than themounting surface for the LED chip.

FIG. 7 is a cross-sectional view showing an example of a chip-typelight-emitting semiconductor device fabricated by a known pouringtechnique. As shown in FIG. 7, the known light-emitting semiconductordevice includes a casing 8; a blue LED 1 placed in the casing 8; aYAG-based luminescent layer 3 surrounding the blue LED 1 in the casing 8and made of a mixture of yellow/yellowish phosphor particles and aresin; and an upper coating 10 covering the YAG-based luminescent layer3 in the casing 8.

FIG. 9 is a SEM micrograph showing a cross-sectional structure of thecoating 10 of the light-emitting semiconductor device in the state shownin FIG. 7. FIG. 10 is a SEM micrograph showing a magnified view of aportion near the casing 8. From an experiment done by the presentinventors, if a luminescent layer is formed by the pouring techniquedescribed above, the coating is divided substantially into theluminescent layer 3 containing a high concentration of the YAG-basedphosphor and the upper coating layer 10 hardly containing the YAG-basedphosphor as shown in the SEM micrographs of FIGS. 7, 9 and 10 during theformation of the coating. This is mainly because of the difference inspecific gravity between the YAG-based phosphor and the resin, whichcauses YAG-based phosphor particles 9 to sediment in the bottom of thecoating by gravity. That is to say, the resultant substantialluminescent layer 3 does not have a structure in which the YAG-basedphosphor particles 9 are scattered throughout the epoxy resin (basematerial) but has a structure in which the YAG-based phosphor particles9 are in contact with each other and unevenly distributed in the basematerial, i.e., sedimented in the bottom of the coating. In this case,the state of being scattered is a state in which the phosphor particlesare evenly dispersed throughout the luminescent layer. The substantialthickness of the luminescent layer 3 is smaller than that of the uppercoating 10 and is 10 to 70 μm.

With respect to the distribution of the YAG-based phosphor particles inthe coating, there appeared “various distributions of thephotoluminescence phosphor can be achieved by controlling, for example,the material which contains the photoluminescence phosphor, formingtemperature, viscosity, the configuration and particle distribution ofthe photoluminescence phosphor . . . ” in Domestic Re-Publication of PCTApplication No. WO98/05078. The possibility of formation of aluminescent layer having a structure in which YAG-based phosphorparticles are evenly scattered in a base material is also suggested.However, an additional examination done by the present inventors provedthat such a structure cannot be formed in reality by the pouringtechnique described above using a YAG-based phosphor and the disclosedresin (e.g., epoxy resin, urea resin or silicon). For confirmation, weobtained a light-emitting device already introduced commercially by theapplicant of Japanese Patent No. 2927279 and estimated thecross-sectional structure of the luminescent layer, to find that thephosphor does not have a structure in which YAG-based phosphor particlesare evenly scatted throughout a base material but has a structure of theluminescent layer as shown in FIG. 9, specifically a structure in whichthe YAG-based phosphor particles are in contact with each other andunevenly distributed in the base material so that the luminescent layeris formed sedimenting in the bottom of the coating. The substantialthickness of the luminescent layer is about 70 μm as shown in the SEMmicrograph of FIG. 9.

In the method for applying a luminescent layer with a photolithographyor transfer process and forming a luminescent layer as a package, theYAG-based phosphor particles also sediment in the bottom of the coatingby gravity during the formation of the luminescent layer. Accordingly,the resultant substantial luminescent layer is not in the state in whichthe YAG-based phosphor particles are scattered throughout the basematerial, resulting in causing uneven distribution of phosphor particlesin the package. If a luminescent layer as a package is formed using ascreen publishing process, the YAG-based phosphor particles lesssediment and come close to the state in which the YAG-based phosphorparticles are scattered throughout the base material, but distributionunevenness of the phosphor particles is still observed. In addition, theresultant luminescent layer exhibits a low luminescence performance.

As described above, in the known light-emitting semiconductor devices,YAG-based phosphor particles are in contact with each other in aluminescent layer and unevenly distributed in a base material in mostcases so that distribution unevenness of the phosphor particles is tendto be observed in the luminescent layer. In summary, with respect to theluminescent layers of the known light-emitting semiconductor devices,the phosphors used are YAG-based phosphors having a substantialthickness of 10 to 70 μm, especially 10 to 30 μm in most cases. Theluminescent layers are each formed by curing a mixture in which aYAG-based phosphor is mixed and dispersed in a resin used as a basematerial (phosphor paste).

Now, a relationship between the structure of the luminescent layer ofthe light-emitting semiconductor device and the color unevenness, and aknown method for suppressing the color unevenness are described.

In a light-emitting semiconductor device using a blue LED and a phosphorin combination, color unevenness in emission of light has been a problemand various measurements have taken to suppress the color unevenness.Most of the measurements are based on fabricating know-how such as theconfiguration and particle size of YAG-based phosphor particles,optimization of particle distribution, selection of a base material forincluding a phosphor, adjustment of viscosity of a phosphor paste andoptimization of drying conditions.

Instead of the fabricating know-how, specific measurements for radicallyimproving the structure of, for example, the luminescent layer have beenproposed. For example, in Japanese Laid-Open Publication No. 11-31845,described is a method using a technique of applying an epoxy resin ontoan LED chip as an adhesive, attaching YAG-based phosphor particles onthe adhesive and then blowing off the YAG-based phosphor particles thathave been excessively attached by splaying gas so that the thickness ofa YAG-based luminescent layer is made uniform and color unevenness oflight emitted by a light-emitting semiconductor device is suppressed. InJapanese Laid-Open Publication No. 2000-208822, described is a methodfor forming a luminescent layer (translucent wavelength-convertinglayer) on the surrounding other than a mounting surface for a blue LEDas a package for covering the blue LED so that the thickness of thepackage from the outer contour surface of the blue LED is madesubstantially uniform in every direction of emission and therefore thethickness of the luminescent layer is made uniform, thereby suppressingcolor unevenness. In Japanese Laid-Open Publication No. 2001-177158,described is a method for polishing and creating the surface of aluminescent layer such that the surface is in parallel with a main lightextracting surface.

Problems to be Solved

As has been described above, since the known light-emittingsemiconductor device uses the YAG-based phosphor as a yellow/yellowishphosphor, the YAG-based phosphor particles sediment in the bottom of acoating by gravity during the formation of the luminescent layer,resulting in that the coating layer is divided into the luminescentlayer in which the phosphor particles are in contact with each other andunevenly distributed in a base material and an upper coating layerhardly containing the YAG-based phosphor. Even if the YAG-based phosphorparticles are not in contact with each other, the luminescent layer hasa structure in which distribution unevenness of the phosphor particlesis large in the base material. The reason of this distributionunevenness is not clear but the difference in specific gravity betweenthe phosphor and the base material is at least one cause of thedistribution unevenness.

As described above, the absolute specific gravity of a Y₃Al₅O₁₂:Ce³⁺phosphor containing no Gd atoms (where the substitution amount of Cewith respect to Y is 0.1 to 2% and the main emission peak wavelength atroom temperature is 530 to 557 nm) is 4.15 to 4.55, though the absolutespecific gravity varies to some extent depending on the composition ofthe phosphor. However, from an evaluation done by the present inventors,the measurement result of the absolute specific gravity of at least a(Y_(0.7)Ge_(0.28)Ce_(0.02))₃Al₅O₁₂ phosphor (whose main emission peakwavelength is 565 nm) in which part of Y is substituted with Gd toobtain excellent yellow/yellowish light is 4.98, and the absolutespecific gravity of every phosphor in which part of the Y₃Al₅O₁₂:Ce³⁺phosphor is substituted with Gd is as high as over 4.65 (see FIG. 48).

It is known that a sulfide phosphor using the (Zn, Cd)S as a phosphorbase can emit yellow/yellowish light having a main emission peak in thewavelength range of about 560 nm or more by containing Cd (see, forexample, “Phosphor Handbook” edited by Phosphor Research Society,Ohmsha, Ltd. p. 248). It is also known that the absolute specificgravity is as low as about 4.13 (see Phosphor Index (Nichia Kagaku KogyoKabushiki Kaisha)). It should be noted that the phosphor not only has alow emission efficiency when irradiated with blue light (excitationlight) but also contains noxious Cd, and therefore the phosphor isdifficult to, for example, fabricate, handle and storage.

Therefore, since the known light-emitting semiconductor devices exhibitdistribution unevenness of phosphor particles in luminescent layers, thedevices have a problem that unevenness is created in emission of lightto cause low fabrication yields. This problem of emission unevenness iscommonly observed among the known light-emitting semiconductor devicesconfigured by using YAG-based phosphors, and also observed inlight-emitting semiconductor devices additionally using red phosphors tocompensate for a shortage of red light, and light-emitting semiconductordevices additionally using green phosphors to enhance luminous efficacy.

The known light-emitting semiconductor devices also have a problem whenviewed from a different point of view. In some of the knownlight-emitting semiconductor devices that include luminescent layers inwhich phosphor particles are in contact with each other and unevenlydistributed, the luminescent layers absorb blue light emitted by blueLEDs and the light is liable to be attenuated, resulting in a problem ofinsufficient luminous flux of white or whitish light obtained by addingthe colors of blue light from the LED and of yellow/yellowish light fromthe YAG-based phosphor together.

A YAG-based phosphor is a blue light excitation phosphor (a phosphorexcited by blue light) that receives blue light between or equal to 410nm and 530 nm emitted by a blue LED to convert the blue light intoyellow/yellowish light between or equal to 550 nm and 600 nm with highconversion efficiency. Accordingly, in a known white-light-emittingsemiconductor device configured by using such a YAG-based phosphor, asmall amount of the YAG-based phosphor with high conversion efficiencyis needed, so that the substantial thickness of the luminescent layer is10 to 70 μm. In many practical light-emitting semiconductor devices, thesubstantial thickness is as small as 10 to 30 μm. If the YAG-basedphosphor particles has a particle size (particle diameter) of about 5 toabout 20 μm and the luminescent layer has a small substantial thickness,the thickness of the luminescent layer is substantially secured by onlyseveral to over ten particles, resulting in that slight surfaceunevenness created in the surface of the luminescent layer has a largeeffect to accentuate unevenness in light emission. On the other hand, ifthe phosphor concentration (phosphor weight/(phosphor weight+resinweight)) of the YAG-based phosphor is set lower than a normal weight of5 to 10 wt %, i.e., lower than 5 wt %, to increase the substantialthickness of the luminescent layer, the light distributioncharacteristics of the light-emitting semiconductor device deteriorate.

To suppress such color unevenness, various kinds of contrivances havebeen made. However, a sufficient solution has yet to be found and therestill exists a problem of low fabrication yields of light-emittingsemiconductor devices. In addition to the color unevenness, thelight-emitting semiconductor device, especially a light-emittingsemiconductor device emitting white or whitish light has a difficulty incontrolling color, i.e., a problem that the color of light emitted bythe device is expressed in a narrow range, because emission peakwavelength of yellow/yellowish light emitted by the YAG-based phosphoris limited in the range from about 550 nm to 590 nm, both inclusive.This is because the color of light emitted by the light-emittingsemiconductor device is determined by adding the colors of blue lightemitted by the blue LEDs and of yellow/yellowish light emitted by theYAG-based phosphors together.

A light-emitting system using such a known light-emitting semiconductordevice has a problem that color unevenness is readily created in thelight-emitting system and a problem that the fabrication yield of thelight-emitting system is low due to the color unevenness. In addition,the low fabrication yield of the light-emitting semiconductor deviceincreases the fabrication cost of the light-emitting system.

DISCLOSURE OF INVENTION

An object of the present invention is suppressing color unevenness in alight-emitting semiconductor device configured by utilizing ablue-light-emitting device and a phosphor in combination so as toprovide a light-emitting semiconductor device or a light-emittingsemiconductor system exhibiting small color unevenness, especially awhite-light-emitting semiconductor device exhibiting a luminous fluxhigher than or equal to that of a known white-light-emittingsemiconductor device utilizing a YAG-based phosphor and ablue-light-emitting device in combination and a light-emitting systemexhibiting small color unevenness and a high luminous flux.

An inventive light-emitting semiconductor device is a light-emittingsemiconductor device including: a blue-light-emitting device having alight-extracting surface and emitting blue light from thelight-extracting surface; and a luminescent layer provided to cover atleast the light-extracting surface of the blue-light-emitting device andincluding a yellow/yellowish phosphor which absorbs blue light emittedby the blue-light-emitting device to emit a yellow/yellowishfluorescence. The yellow/yellowish phosphor is a silicate phosphorcontaining, as a main component, at least one type of a compoundexpressed by the chemical formula:(Sr_(1−a1−x)Ba_(a1)Ca_(b1)Eu_(x))₂SiO₄(where 0≦a1≦0.3, 0≦b1≦0.8 and 0<x<1).

Considering the fact that the blue-light-emitting device can achieve alight-emitting semiconductor device emitting excellent white light, theblue-light-emitting device preferably emits light having a main emissionpeak in the wavelength range greater than 430 nm and less than or equalto 500 nm, more preferably in the wavelength range from 440 nm to 490nm, both inclusive, and still more preferably in the wavelength rangefrom 450 nm to 480 nm, both inclusive. The yellow/yellowish phosphorpreferably emits a fluorescence having a main emission peak in thewavelength range from 550 nm to 600 nm, both inclusive, more preferablyin the wavelength range from 560 nm to 590 nm, both inclusive, and stillmore preferably in the wavelength range from 565 nm to 585 nm, bothinclusive.

In terms of crystal stability to heat in the phosphor, heat resistancein light-emitting characteristics, luminous intensity ofyellow/yellowish emission and color of light, the values a1, b1 and x inthe chemical formula are preferably in the ranges 0<a1≦0.2, 0<b1≦0.7 and0.05<x<0.1, respectively, more preferably in the ranges 0<a1≦0.15,0<b1≦0.6 and 0.01<x<0.05, respectively, and most preferably in theranges 0.01≦a1≦0.1, 0.001≦b1≦0.05 and 0.01<x≦0.02, respectively.

FIG. 8 is a graph showing excitation-light spectra and emission spectraof a silicate phosphor and a YAG-based phosphor. As shown in FIG. 8, thesilicate phosphor is a yellow/yellowish phosphor which has anexcitation-light peak around 250 to 300 nm and absorbs light in the widewavelength range of 100 to 500 nm to emit a yellow/yellowishfluorescence having an emission peak in the wavelength range of 550 to600 nm, i.e., from yellow-green, yellow to orange ranges. Accordingly,if the yellow/yellowish phosphor is combined with theblue-light-emitting device, the resultant light-emitting semiconductordevice emits light by adding the color of the fluorescence emitted bythe yellow/yellowish phosphor to the color of the blue light emitted bythe blue-light-emitting device.

Now, a relationship among the composition range, the crystal structureand the color of emitted light of the silicate phosphor, thecharacteristics of the silicate phosphor emitting yellow/yellowishlight, for example, are described in further detail. As a first case, ifboth of the values a1 and b1 in the chemical formula of the silicatephosphor are close to zero, the silicate phosphor is likely to have amonoclinic structure or a crystal structure in which an orthorhombicsystem and a monoclinic system are mixed. As a second case, if the valuea1 deviates to larger values from the most preferable range and thevalue b1 is close to zero, the crystal field around Eu²⁺ ions is weak.As a third case, if the value a1 is close to zero and the value b1deviates to larger values from the most preferable range, the silicatephosphor is likely to have a monoclinic structure. As a fourth case, ifboth of the values a1 and b1 deviate to larger values from therespective most preferable ranges and the value 1−a1−b1−x is close tozero, the silicate phosphor is likely to have a hexagonal structure. Inany of the first through fourth cases, the silicate phosphor might be agreener phosphor and emit light with low color purity for yellow. If thevalue x deviates to smaller values from the most preferable range, theconcentration of Eu²⁺ luminescent centers is low, resulting in lowluminous intensity of the silicate phosphor. If the value x deviates tolarger values from the most preferable range, the luminous intensity islow because of concentration quenching (of luminescence) orself-absorption caused by Eu²⁺ ions and, moreover, thermal quenchingthat the luminous intensity decreases as the ambient temperature of thesilicate phosphor increases might occur.

Comparison in excitation spectrum between the silicate phosphor and theYAG-based phosphor shown in FIG. 8 as an example shows that the silicatephosphor is a phosphor having a low luminous efficacy (e.g., having aluminous intensity only half of that of the YAG-based phosphor under470-nm excitation) when excited by blue light in the wavelength rangegreater than 430 nm and less than or equal to 500 nm. Therefore, alarger amount of a phosphor is used in the case where the silicatephosphor is used than in the case where the YAG-based phosphor is used,in order to obtain the same color of light with a white-light-emittingsemiconductor device that emits white light by adding the colors of theblue light from the blue LED and the yellow light from theyellow/yellowish phosphor together. Accordingly, the luminescent layeris relatively thick if the silicate phosphor is used. As a result, thephosphor is less affected by the unevenness created in the surface ofthe luminescent layer, so that variation in thickness of the luminescentlayer becomes substantially small, thus obtaining a light-emittingsemiconductor device which emits light with small color unevenness.

The blue-light-emitting device is a device selected from the groupconsisting of a blue-light-emitting diode, a laser diode, a surfaceemitting laser diode, a resonant cavity light emitting diode, aninorganic electroluminescence device and an organic electroluminescencedevice. In terms of increasing the output power and the lifetime of thelight-emitting semiconductor device, diodes such as a light-emittingdiode, a laser diode, a surface emitting laser diode and a resonantcavity light emitting diode are superior to other devices.

The Ca mole fraction b1 of the yellow/yellowish phosphor is preferably amole fraction b2 in the range 0≦b2≦0.6.

In terms of crystal stability to heat in the phosphor, heat resistancein light-emitting characteristics, luminous intensity ofyellow/yellowish emission and color of light, the mole fraction b2 ispreferably in the range 0<b2≦0.4, more preferably in the range 0<b2≦0.3,and most preferably in the range 0.001≦b2≦0.05.

The orthorhombic silicate phosphor within any of the composition rangesemits yellow/yellowish light highly efficiently with high color purityfor yellow when excited the blue excitation light. As a result, lightemitted by the light-emitting semiconductor device not only has a highluminous flux but also is white or whitish light with a high colorpurity for white.

In the light-emitting semiconductor device, the blue-light-emittingdevice may be a blue-light-emitting inorganic device made of asemiconductor selected from the group consisting of a galliumnitride-based compound semiconductor, a zinc selenide semiconductor anda zinc oxide semiconductor. Such a blue-light-emitting inorganic device,especially a blue-light-emitting device including a light-emitting layermade of a gallium nitride-based compound semiconductor, exhibits a highluminous efficacy. Accordingly, if such a blue-light-emitting inorganicdevice, especially a blue-light-emitting device including alight-emitting layer made of a gallium nitride-based compoundsemiconductor, is combined with the silicate phosphor, a light-emittingsemiconductor device emitting light with a high luminous flux isobtained.

In the light-emitting semiconductor device, the color of light emittedby the light-emitting semiconductor device may have an emissionchromatically point (x, y) in the ranges 0.21≦x≦0.48 and 0.19≦y≦0.45,respectively, in a CIE chromaticity diagram.

This chromatically range includes a large part of the white range.Accordingly, if the color of light emitted by the light-emittingsemiconductor device is set within the chromatically range, ahighly-demanded white-light-emitting semiconductor device is obtained.

A red/reddish phosphor having an emission peak in the red/reddishwavelength range greater than 600 nm and less than or equal to 660 nmmay be provided.

Then, the red/reddish phosphor compensates for a red spectrum whichcannot be compensated by the yellow/yellowish phosphor alone, so thatthe light emitted by the light-emitting semiconductor device includes awide range of the red spectrum.

Further, a green/greenish phosphor having a main emission peak in thegreen/greenish wavelength range greater than or equal to 500 nm and lessthan 550 nm may be provided.

Then, the green/greenish phosphor compensates for a green spectrum whichhas a high luminous efficacy and which cannot be compensated by theyellow/yellowish phosphor alone, so that the light emitted by thelight-emitting semiconductor device includes a wide range of the greenspectrum. Accordingly, the light emitted by the light-emittingsemiconductor device is white light with a high luminous efficacy withrespect to white. The red/reddish phosphor and the green/greenishphosphor may be combined with the yellow/yellowish phosphor.

The green/greenish phosphor is preferably a silicate phosphorcontaining, as a main component, a compound expressed by the chemicalformula:(Sr_(1−a3−b3−x)Ba_(a3)Ca_(b3)Eu_(x))₂SiO₄(where 0≦a3≦1, 0≦b3≦1 and 0<x<1).Then, the composition and crystal structure of the green/greenishphosphor are made similar to those of the silicate phosphor emittingyellow/yellowish light. Accordingly, the color unevenness in thelight-emitting semiconductor device including the green/greenishphosphor becomes relatively small, and in addition, a new technique isno more required in a process for fabricating the light-emittingsemiconductor device, resulting in a simple fabrication process.

The luminescent layer may include a plurality of such silicate phosphormade of compounds mutually differing in composition and each emittingyellow/yellowish light having a main emission peak in the wavelengthrange from 550 nm to 600 nm, both inclusive. Then, it is possible tocontrol the color of white light obtained by adding the colors of bluelight emitted by the blue-light-emitting device and of yellow/yellowishlight emitted by the silicate phosphors.

The luminescent layer preferably includes a translucent resin as a basematerial; and the yellow/yellowish phosphor is preferably present in theform of dispersed particles in the base material.

Since such a luminescent layer includes substantially neither lightabsorption factor nor light scattering factor, the luminescent layerexhibits improved light transmissivity. Accordingly, the blue light fromthe blue-light-emitting device either passes through the luminescentlayer without being absorbed and attenuated or contributes to excitationof the phosphor. In addition, since the luminescent layer is in thestate that a larger part of the surfaces of the phosphor particles canbe irradiated with the blue light, the cross-sectional area of thephosphor particles to be excited increases substantially, so that thephosphor particles in the luminescent layer emit light effectively.

Moreover, since the phosphor in which the phosphor particles aredispersed has its substantial thickness increased, the influence ofvariation in thickness of the luminescent layer becomes small, thusreducing emission unevenness caused by the variation in thickness of theluminescent layer. As the translucent base material, a resin or likematerials may be used. As the resin, a resin such as an epoxy resin, anacrylic resin, a polyimide resin, a urea resin or a silicone resin maybe used, and an epoxy resin or a silicone resin is preferably used.

The luminescent layer may be made by forming (sintering) the silicatephosphor.

The light-emitting semiconductor device preferably has a structure inwhich blue light emitted by the blue-light-emitting device passesthrough the luminescent layer so that the color of the fluorescenceemitted by the phosphor is added to the color of the blue light, therebyemitting white light.

The inventive light-emitting semiconductor device may have any of thestructures including the following members.

A first structure is a structure in which a substrate is furtherprovided, the blue-light-emitting device is flip-chip mounted on thesubstrate, and the luminescent layer functions as a molding resin formolding the blue-light-emitting device.

In such a case, the substrate preferably includes a Zener diode.

A second structure is a structure in which a mount lead with a cup isfurther provided, the blue-light-emitting device is mounted in the cup,and the luminescent layer is provided within the cup.

A third structure is a structure in which a casing for placing theblue-light-emitting device therein is further provided, and theluminescent layer may be provided within the casing.

The light-emitting semiconductor device with such structures isimplemented as a light-emitting semiconductor device emitting whitelight with a high luminous flux. The light-emitting semiconductor devicecan be fabricated through a relatively simple process, so that thefabrication yield enhances.

Among the light-emitting semiconductor devices having the first throughthird structures, the light-emitting semiconductor device with the firststructure has the characteristic that the color unevenness is inherentlysmaller than those in the other light-emitting semiconductor deviceswith the second and third structures. Therefore, the light-emittingsemiconductor device with the first structure is preferably used becausethe color unevenness in the light-emitting semiconductor device isfurther reduced so that the production yield further enhances.

The luminescent layer preferably has a substantial thickness in therange from 50 μm to 1000 μm, both inclusive, where the light extractionsurface of the blue-light-emitting device is located.

If the substantial thickness of the luminescent layer is set within therange from 50 μm to 1000 μm, both inclusive, and more preferably withinthe range from 100 μm to 700 μm, both inclusive, the cross-sectionalarea of the silicate phosphor to be excited by the blue light increases,as compared to the case of the known YAG phosphor. Accordingly, theluminous intensity of the yellow light emitted by the silicate phosphorincreases, and the light-emitting semiconductor device emits white lightwith excellent color tone by adding the colors the yellow light and ofthe blue light emitted by the blue-light-emitting device together.Moreover, as described above, since the luminescent layer hassubstantially no light absorption and attenuation factor, blue lightemitted by a blue-light-emitting device either passes through theluminescent layer without being absorbed and attenuated or contributesto excitation of the phosphor, thus increasing the luminous intensity ofthe yellow light emitted by the silicate phosphor. Accordingly, if anoptimum phosphor concentration (weight ratio between resin andphosphor:phosphor weight/(phosphor weight+resin weight)) is selected,the light-emitting semiconductor device emits white light with aluminous flux higher than that of the known light-emitting semiconductordevice using the YAG-based phosphor.

As compared to the known light-emitting semiconductor device using theYAG-based phosphor, the substantial thickness of the luminescent layerincrease largely. Therefore, even if surface unevenness in thesubstantial luminescent layer is large to some extent, the thickness ofthe entire luminescent layer is less affected by the surface unevenness,so that apparent thickness variation is reduced. As a result, emissionunevenness caused by the variation in thickness of the luminescent layeris also reduced.

If the substantial thickness of the luminescent layer is smaller thanthe preferable thickness ranges, the cross-sectional area of thesilicate phosphor to be excited by the blue light is small so that thesubstantial luminous efficacy of the phosphor is low. Accordingly, thelight-emitting semiconductor device emits bluer light in which theemission from the blue-light-emitting device is dominant, resulting inthat there may be cases where white light with excellent color tonecannot be obtained or a high luminous flux cannot be obtained. On theother hand, if the substantial thickness is larger than the thicknessranges, the cross-sectional area of the silicate phosphor to be excitedby the blue light is large so that the substantial luminous efficacy ofthe phosphor is high but most of the blue light is absorbed in thephosphor to be converted into yellow/yellowish light. Accordingly, thelight-emitting semiconductor device emits yellower light in which theyellow/yellowish emission from the silicate phosphor is dominant,resulting in that there may be cases where white light with excellentcolor tone cannot be obtained. Further, there may also be cases where ahigh luminous flux cannot be obtained because the phosphor particles areprone to be partially in contact with each other to cause blue lightfrom the blue-light-emitting device to be more and more absorbed andattenuated.

In the known light-emitting semiconductor device using the YAG-basedphosphor, if the luminescent layer is made thick as in the inventivelight-emitting semiconductor device, the YAG-based phosphor has anextremely high luminous efficacy when irradiated with blue light under aphosphor concentration condition (10 to 80 wt. %) in the luminescentlayer including a general silicate phosphor. Accordingly, only yelloweremission in which light emitted by the YAG-based phosphor is dominant isobtained and the luminous flux also decreases. If the phosphorconcentration is reduced so as to secure a desired color of light and toobtain such a thick luminescent layer, YAG-based phosphor particles areliable to disperse unevenly in a base material (a resin). This resultsin that the chromaticity and the luminance of the resultant emissionvary largely and, in addition, the light distribution characteristicsare prone to deteriorate. For this reason, the light-emittingsemiconductor device has only a small commercial value.

As is explicitly shown in the case where phosphor particles are unevenlydistributed in the base material and which is herein described using themicrographs in FIGS. 9 and 10, an average thickness of the luminescentlayer with which the presence of the phosphor particles in the basematerial can be visually observed clearly by a cross-sectionalobservation of the light-emitting semiconductor device with an electronmicroscope of a magnification from ×50 to ×1000 is defined as a“substantial thickness of the luminescent layer”.

The upper surface of a portion of the luminescent layer located at leaston the light-extracting surface of the blue-light-emitting device ispreferably flat and substantially parallel to the light-extractingsurface.

Most of the blue-light-emitting devices are fabricated to have flatlight-extracting surfaces (especially flat main light-extractingsurfaces) because of the easiness of the fabrication. Therefore, if thesurface of the luminescent layer is formed to be parallel to thelight-extracting surface (especially the main light-extracting surface),the distance from the outer contour surface of the light-extractingsurface to the outer contour surface of the luminescent layer, i.e., thethickness of the luminescent layer, is made substantially uniform inalmost every part of the luminescent layer located on thelight-extracting surface, so that the variation in thickness of theluminescent layer is further suppressed and, therefore, emissionunevenness of the light-emitting semiconductor device is reduced.

The blue-light-emitting device may be provided in a plural presence andthe luminescent layer may be provided to cover respective light-emittingsurfaces of the plurality of blue-light-emitting devices.

An inventive light-emitting system is a light-emitting system including:a blue-light-emitting device emitting blue light; a luminescent layerincluding a yellow/yellowish phosphor which absorbs blue light emittedby the blue-light-emitting device to emit a yellow/yellowishfluorescence; and a supporter for supporting the blue-light-emittingdevice and the luminescent layer. The yellow/yellowish phosphor is asilicate phosphor containing, as a main component, at least one type ofa compound expressed by the chemical formula:(Sr_(1−a1−b1−x)Ba_(a1)Ca_(b1)Eu_(x))₂SiO₄(where 0≦a1≦0.3, 0≦b1≦0.8 and 0<x<1).

The inventive light-emitting semiconductor device including theblue-light-emitting device and the luminescent layer exhibits smallcolor unevenness, resulting in a high production yield and a lowproduction cost. Accordingly, if a light-emitting system is configuredusing the light-emitting semiconductor device, the light-emittingsemiconductor system not only has reduced color unevenness but also isfabricated at a low cost. Moreover, since such a light-emittingsemiconductor device exhibits a luminous flux higher than that of theknown light-emitting semiconductor device using the YAG-based phosphor,the luminous flux of the light-emitting system enhances.

The blue-light-emitting device may be provided in a plural presence andthe luminescent layer may be provided to cover respective light-emittingsurfaces of the plurality of blue-light-emitting devices.

In this description, various kinds of display systems usinglight-emitting semiconductor devices (e.g., LED information displayterminals, LED traffic lights, LED stoplights of vehicles, and LEDdirectional lights) and various kinds of lighting systems (e.g., LEDinterior/exterior lights, courtesy LED lights, LED emergency lights, andLED surface emitting sources) are broadly defined as light-emittingsystems.

An inventive method for fabricating a luminescent layer of alight-emitting semiconductor device is a method for fabricating alight-emitting semiconductor device including: a blue-light-emittingdevice emitting light having a main emission peak in the wavelengthrange greater than 430 nm and less than or equal to 500 nm; and aluminescent layer including a yellow/yellowish phosphor which absorbsblue light emitted by the blue-light-emitting device to emit afluorescence having a main emission peak in the wavelength range from550 nm to 600 nm, both inclusive. The method includes the steps of a)covering at least a light-extracting surface of the blue-light-emittingdevice with a phosphor paste including the yellow/yellowish phosphorwhich has an absolute specific gravity in the range from 3.0 to 4.65,both inclusive, and emits light having a main emission peak in thewavelength range from 560 nm to 600 nm, both inclusive, at roomtemperature, and with a resin which has an absolute specific gravity inthe range greater than or equal to 0.8 and less than or equal to theabsolute value of the yellow/yellowish phosphor; and b) curing thephosphor paste, thereby forming the luminescent layer. In the step a), aphosphor including, as a base material, a compound containing at leastone element selected from the group consisting of Mg, Ca, Sr, Ba, Sc, Y,lanthanoid, Ti, Zr, Hf, V, Nb, Ta, Mo, W, Zn, B, Al, Ga, In, Si, Ge, Snand P and at least one element selected from the group consisting of O,S, Se, F, Cl and Br is used as the yellow/yellowish phosphor. Thecomponent does not include an element having a large specific gravitysuch as Cd.

In this manner, if a yellow/yellowish phosphor having an absolutespecific gravity smaller than that of a YAG-based phosphor is used inthe method for fabricating the luminescent layer by curing the phosphorpaste including the yellow/yellowish phosphor and the resin, thedifference in specific gravity between the resin (generally having anabsolute value smaller than that of the phosphor except for specialcases) and the phosphor is reduced. Thus, the phosphor is less liable tosediment in the phosphor paste by gravity before or during the curing ofthe phosphor paste, resulting in that the resultant luminescent layerhas a structure in which the phosphor particles are dispersedsubstantially evenly throughout the resin or a like structure. Inaddition, since a phosphor containing no noxious Cd, preferably aphosphor including an oxide, is used, fabrication, handling, storage,control and the like therefor are easy.

In the step a), a yellow/yellowish phosphor having a particle size inthe range from 0.5 μm to 30 μm, both inclusive, may be used.

Then, it is possible to obtain a luminescent layer exhibiting a highluminous intensity and having a structure in which phosphor particlesare dispersed.

In the step a), a silicate phosphor containing, as a main component, atleast one type of a compound expressed by the chemical formula(Sr_(1−a1−b1−x)Ba_(a1)Ca_(b1)Eu_(x))₂SiO₄(where 0≦a1≦0.3, 0≦b1≦0.8 and 0<x<1)may be used as the yellow/yellowish phosphor.

It should be noted that the values a1, b1 and x are preferably in theranges 0<a1≦0.2, 0≦b1≦0.7 and 0.005<x<0.1. A silicate phosphor having acomposition within the preferable ranges and emitting a fluorescencewith a main emission peak in the wavelength range from 560 nm to 600 nm,both inclusive, has an absolute specific gravity smaller than that of aYAG-based phosphor in general, i.e., in the range from 3.0 to 4.65, bothinclusive. In addition, the silicate phosphor also serves as ayellow/yellowish phosphor which emits yellow/yellowish light whenexcited by blue light. Therefore, if a luminescent layer is formed usingthe silicate phosphor in combination with a resin (e.g., an epoxy resin)having an absolute specific gravity in the range greater than or equalto 0.8 and less than or equal to the absolute value of theyellow/yellowish phosphor, the luminescent layer is implementable as aluminescent layer exhibiting a high luminous intensity and having astructure in which phosphor particles are dispersed in an actualprocess. In the case where the value a1 is larger than the preferablerange, as the value a1 increases, the absolute specific gravity of thesilicate phosphor increases so that the phosphor particles are moreliable to sediment in the phosphor paste and, as a result, it might beimpossible to obtain a luminescent layer with a structure in which thephosphor particles are dispersed. This action is due to the fact that aBa atom is heavier than a Sr atom.

Now, a relationship among the composition, the absolute specificgravity, and the main emission peak wavelength of a YAG-based phosphoris simply described. Though the absolute gravity of a YAG-based phosphorvaries largely depending on the composition, especially the amount of Gdsubstituting Y, a YAG-based phosphor having a main emission peak in thewavelength range from 560 nm to 600 nm, both inclusive, under roomtemperature has a large substitution amount of Gd in general, resultingin having an absolute gravity greater than 4.55, greater, moreover, than4.60, 4.65, and even exceeding 4.7. That is to say, such a YAG-basedphosphor is heavy.

Ultra-fine particles including primary particles having an averageparticle size in the range from 1 nm to 100 nm, both inclusive, may beincluded in the phosphor paste and a luminescent layer may be formed bycuring the phosphor paste. The sedimentation speed of the ultra-fineparticles in a resin is extremely low and is almost zero. Therefore, inthis manner, the ultra-fine particles suspended in the resin act in sucha manner as preventing the sedimentation of the yellow/yellowishphosphor, so that the sedimentation speed of the yellow/yellowishphosphor decreases. As a result, a luminescent layer with a structure inwhich phosphor particles are dispersed is easily obtained.

In addition, the device may be configured such that blue light emittedby a blue-light-emitting diode passes through the luminescent layer sothat the color of a (yellow/yellowish, red/reddish or green/greenish)fluorescence emitted by the phosphor is added to the color of the bluelight, thereby emitting white light. Then, the colors of the blue lightand the (yellow/yellowish, red/reddish or green/greenish) fluorescenceemitted by the phosphor are added together as intended, therebyobtaining white light.

Examples of methods for suppressing the sedimentation of the phosphorparticles while the phosphor paste is curing include the followingmethods.

A first method is a method for fabricating a light-emittingsemiconductor device including the steps of: a) covering alight-extracting surface of a blue-light-emitting device with a phosphorpaste including a resin and phosphor particles; and b) curing thephosphor paste while applying a vibration to the phosphor paste.

A second method is a method for fabricating a light-emittingsemiconductor device including the steps of: a) covering alight-extracting surface of a blue-light-emitting device with a phosphorpaste including a resin and phosphor particles; and b) curing thephosphor paste while turning over the phosphor paste.

A third method is a method for fabricating a light-emittingsemiconductor device including the steps of a) covering alight-extracting surface of a blue-light-emitting device with a phosphorpaste including a resin and phosphor particles; and b) curing thephosphor paste, wherein the steps a) and b) are performed a plurality oftimes.

A fourth method is a method for fabricating a light-emittingsemiconductor device including the steps of a) covering alight-extracting surface of a blue-light-emitting device with a phosphorpaste including a resin and phosphor particles and having a viscosity inthe range from 1 Pa·S to 100 Pa·S, both inclusive; and b) curing thephosphor paste, wherein the steps a) and b) are performed a plurality oftimes.

A fifth method for fabricating a light-emitting semiconductor deviceincluding the steps of a) covering a light-extracting surface of ablue-light-emitting device with a phosphor paste including a resin andphosphor particles; and b) curing the phosphor paste with ultravioletradiation.

A sixth method for fabricating a light-emitting semiconductor deviceincluding the steps of a) covering a light-extracting surface of ablue-light-emitting device with a phosphor paste including a resin andphosphor particles; and b) curing the phosphor paste while agitating thephosphor paste.

Examples of methods for obtaining a light-emitting semiconductor devicewith a structure in which a maximum amount of phosphor particles aremade close to the blue-light-emitting device include the followingmethods.

A first method is a method including the steps of: a) covering at leasta light-extracting surface of a blue-light-emitting device which emitslight having a main emission peak in the wavelength range greater than430 nm and less than or equal to 500 nm, with a first phosphor pasteincluding a base material of a translucent resin and phosphor particlesincluding a yellow/yellowish phosphor; b) covering the first phosphorpaste with a second phosphor paste including at least a translucentresin and containing a yellow/yellowish phosphor at a concentrationlower than that in the first phosphor paste, after the step a) has beenperformed; and c) curing the first and second phosphor pastes. In thestep a), as the yellow/yellowish phosphors, a silicate phosphor which isa yellow/yellowish phosphor absorbing light emitted by theblue-light-emitting device to emit light having a main emission peak inthe wavelength range from 550 nm to 600 nm, both inclusive, and whichcontains, as a main component, at least one type of a compound expressedby the chemical formula(Sr_(1−a1−b1−x)Ba_(a1)Ca_(b1)Eu_(x))₂SiO₄(where 0≦a1≦0.3, 0≦b1≦0.8 and 0<x<1)is used.

A second method is a method including the steps of: a) attachingphosphor particles including a yellow/yellowish phosphor to at least alight-extracting surface of a blue-light-emitting device which emitslight having a main emission peak in the wavelength range greater than430 nm and less than or equal to 500 nm; b) covering at least thelight-extracting surface of the blue-light-emitting device with atranslucent resin, after the step a) has been performed; and c) curingthe resin. In the step a), as the yellow/yellowish phosphor, a silicatephosphor which is a yellow/yellowish phosphor absorbing light emitted bythe blue-light-emitting device to emit light having a main emission peakin the wavelength range from 550 nm to 600 nm, both inclusive, and whichcontains, as a main component, at least one type of a compound expressedby the chemical formula(Sr_(1−a1−b1−x)Ba_(a1)Ca_(b1)Eu_(x))₂SiO₄(where 0≦a1≦0.3, 0≦b1≦0.8 and 0<x<1)is used.

In this method, in the step a), the yellow/yellowish phosphor particlesmay be sprinkled on the blue-light-emitting device. Alternatively, inthe step a), the blue-light-emitting device may be immersed in asuspension containing phosphor particles including a yellow/yellowishphosphor and a volatile solvent, and then the solvent may be evaporated.

A third method is a method including the steps of: a) covering at leasta light-extracting surface of a blue-light-emitting device which emitslight having a main emission peak in the wavelength range greater than430 nm and less than or equal to 500 nm, with a phosphor paste includinga translucent resin and phosphor particles, which includes ayellow/yellowish phosphor and to whose surfaces positively chargedsubstances are attached; and b) curing the phosphor paste. In the stepa), as the yellow/yellowish phosphor, a silicate phosphor which is ayellow/yellowish phosphor absorbing light emitted by theblue-light-emitting device to emit light having a main emission peak inthe wavelength range from 550 nm to 600 nm, both inclusive, and whichcontains, as a main component, at least one type of a compound expressedby the chemical formula(Sr_(1−a1−b1−x)Ba_(a1)Ca_(b1)Eu_(x))₂SiO₄(where 0≦a1≦0.3, 0≦b1≦0.8 and 0<x<1)is used.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a vertical cross-sectional view showing a first exemplarylight-emitting semiconductor device according to a first embodiment ofthe present invention.

FIG. 2 is a vertical cross-sectional view showing a second exemplarylight-emitting semiconductor device according to the first embodiment ofthe present invention.

FIG. 3 is a vertical cross-sectional view showing a third exemplarylight-emitting semiconductor device according to the first embodiment ofthe present invention.

FIG. 4 is a perspective view schematically showing a configuration of adesk-lamp-type lighting system as a first exemplary light-emittingsystem according to a second embodiment of the present invention.

FIG. 5 is a perspective view schematically showing a configuration of animage displaying system as a second exemplary light-emitting systemaccording to the second embodiment of the present invention.

FIG. 6 is a perspective view schematically showing a configuration of apattern displaying system as a third exemplary light-emitting systemaccording to the second embodiment of the present invention.

FIG. 7 is a cross-sectional view showing an example of a chip-typelight-emitting semiconductor device fabricated by a known pouringtechnique.

FIG. 8 is a graph showing excitation-light spectra and emission spectraof a silicate phosphor and a YAG-based phosphor.

FIG. 9 is a SEM micrograph showing a cross-sectional structure of acoating of the light-emitting semiconductor device in the state shown inFIG. 7.

FIG. 10 is a SEM micrograph showing a magnified view of a portion near acasing.

FIG. 11 is a graph showing a luminous intensity (main emission peakintensity) of a silicate phosphor after primary firing and a luminousintensity (main emission peak intensity) of a silicate phosphor aftersecondary firing, as functions of a primary firing temperature.

FIGS. 12( a) through 12(d) are cross-sectional views showing respectiveprocess steps for fabricating a light-emitting semiconductor device of afirst concrete example.

FIGS. 13( a) and 13(b) are respectively a top view and a cross-sectionalview showing a light-emitting semiconductor device formed by afabrication method of the first concrete example.

FIGS. 14( a) through 14(c) are cross-sectional views showing first-halfstages of a process for fabricating a light-emitting semiconductordevice of a second concrete example.

FIGS. 15( a) and 15(b) are cross-sectional views showing the latter halfof the process for fabricating the light-emitting semiconductor deviceof the second concrete example.

FIGS. 16( a) through 16(c) are cross-sectional views showing first-halfstages of a process for fabricating a light-emitting semiconductordevice of a third concrete example.

FIGS. 17( a) and 17(b) are plan views respectively showing two methodsfor applying ultrasonic vibration in a first concrete example of amethod for fabricating a light-emitting semiconductor device.

FIGS. 18( a) and 18(b) are plan views respectively showing two methodsfor applying ultrasonic vibration in the first concrete example of themethod for fabricating a light-emitting semiconductor device.

FIGS. 19( a) and 19(b) are cross-sectional views showing a method forturning over a mold in the first concrete example (transfer technique)of the method for fabricating a light-emitting semiconductor device.

FIGS. 20( a) and 20(b) show the respective states in turning over themold in the step shown in FIG. 15( a) in the second concrete example ofthe process for fabricating the light-emitting semiconductor device.

FIGS. 21( a) through 21(d) are cross-sectional views for use incomparison between a known white-light-emitting device in which phosphorparticles sediment and a white-light-emitting device in which phosphorparticles are evenly dispersed in a resin.

FIG. 22 is a cross-sectional view showing a preferred concrete exampleof a phosphor paste discharging apparatus.

FIG. 23 is an X-ray diffraction pattern showing a result of an X-raydiffraction analysis performed on a silicate phosphor and also showingthe relationship between the diffraction angle and the X-ray diffractionintensity.

FIG. 24 is a graph showing a particle-size distribution in the silicatephosphor observed with a particle size analyzer.

FIG. 25 is a graph showing a result of evaluation performed on emissionof the silicate phosphor through integration using an integratingsphere.

FIGS. 26( a) and 26(b) are X-ray diffraction patterns respectivelyshowing a (Sr_(0.98)Eu_(0.02))₂SiO₄ phosphor containing neither Ca norBa and a publicly known monoclinic Sr₂SiO₄ compound.

FIGS. 27( a) and 27(b) are X-ray diffraction patterns respectivelyshowing a (Sr_(0.93)Ba_(0.5)Eu_(0.02))₂SiO₄ phosphor containing no Caand containing 5 at. % Ba in terms of substitution amount and a publiclyknown orthorhombic Sr₂SiO₄ compound.

FIGS. 28( a) and 28(b) are X-ray diffraction patterns respectivelyshowing a (Ba_(0.98)Eu_(0.02))₂SiO₄ phosphor containing neither Ca norSr and a known orthorhombic Ba₂SiO₄ compound.

FIGS. 29( a) and 29(b) are X-ray diffraction patterns respectivelyshowing a (Ca_(0.38)Ba_(0.60)Eu_(0.02))₂SiO₄ phosphor containing 38 at.% Ca and 60 at. % Ba and a known hexagonal Ba_(0.3)Ca_(0.7)SiO₄compound.

FIGS. 30( a) and 30(b) are X-ray diffraction patterns respectivelyshowing a (Ca_(0.98)Eu_(0.02))₂SiO₄ phosphor containing neither Sr norBa and a publicly known monoclinic Ca₂SiO₄ compound.

FIGS. 31( a) and 31(b) are X-ray diffraction patterns respectivelyshowing a (Sr_(0.84)Ba_(0.14)Eu_(0.2))₂(Si_(0.8)Ge_(0.2))O₄ phosphor inwhich part of Si is substituted with Ge and a publicly knownorthorhombic Sr₂SiO₄ compound.

FIG. 32 is a graph showing emission spectra of(Sr_(0.98−a3)Ba_(a3)Eu_(0.02))₂SiO₄ phosphors having different Basubstitution amounts (a3).

FIG. 33 is a graph showing emission spectra of(Ca_(b3)Sr_(0.93−b3)Ba_(0.05)Eu_(0.02))₂SiO₄ phosphors containing 5 at.% Ba in terms of substitution amount and having different Casubstitution amounts (b3).

FIG. 34 is a graph showing emission spectra of(Ca_(b3)Ba_(0.98−b3)Eu_(0.02))₂SiO₄ phosphors having different Casubstitution amounts (b3).

FIG. 35 is a graph showing emission spectra of(Ca_(0.19)Sr_(0.55)Ba_(0.24)Eu_(0.02))₂SiO₄ phosphor in which the Casubstitution amount (b3) is 19 at. % and the Ba substitution amount (a3)is 24 at. %.

FIG. 36 is a graph showing a dependence of the main emission peakwavelength on the Ba substitution amount (a3) in a(Sr_(0.98−a3)Ba_(a3)Eu_(0.02))₂SiO₄ phosphor (a silicate phosphor).

FIG. 37 is a graph showing a dependence of the main emission peakwavelength on the Ca substitution amount (b3) in a(Ca_(b3)Sr_(0.93−b3)Ba_(0.05)Eu_(0.02))₂SiO₄ phosphor (a silicatephosphor).

FIG. 38 is a graph showing a dependence of the main emission peakwavelength on the Ca substitution amount (b3) in a(Ca_(b3)Ba_(0.98−b3)Eu_(0.02))₂SiO₄ phosphor (a silicate phosphor).

FIG. 39 is a graph showing an emission spectrum of a(Sr_(0.84)Ba_(0.14)Eu_(0.02))₂(Si_(0.8)Ge_(0.2))O₄ phosphor in whichpart of Si is substituted with Ge, for reference.

FIG. 40 is a graph showing emission spectra of (Sr_(1−x)Eu_(x))₂SiO₄phosphors having mutually different Eu concentrations (x) for reference.

FIG. 41 is a graph showing emission spectra of(Sr_(0.95−x)Ba_(0.05)Eu_(x))₂SiO₄ phosphors for reference.

FIG. 42 is a graph showing respective dependences of the main emissionpeak wavelengths on the Eu concentrations of a (Sr_(1−x)Eu_(x))₂SiO₄phosphor and a (Sr_(0.95−x)Ba_(0.05)Eu_(x))₂SiO₄ phosphor.

FIG. 43 is a graph showing an example of a relationship betweenluminescence characteristic of a phosphor and luminescent-centerconcentration.

FIG. 44 is a graph showing a relationship between phosphor weightpercent and luminance.

FIG. 45 is a graph showing a relationship between phosphor concentrationand total luminous flux.

FIG. 46 is a graph showing a relationship between phosphor concentrationand total radiant flux.

FIG. 47 is a graph showing a relationship between phosphor concentrationand chromaticity (value x).

FIG. 48 is a graph showing a relationship between absolute specificgravity and main emission peak wavelength for a YAG-based phosphor and asilicate phosphor.

FIG. 49 is a plan view showing a state of a wafer with a plurality ofZener diodes in connecting blue LEDs to the respective Zener diodes.

FIGS. 50( a) through 50(c) are cross-sectional views showing respectiveprocess steps in a first example of a fabrication method according to athird embodiment.

FIGS. 51( a) through 51(c) are cross-sectional views showing respectiveprocess steps in a second example of the fabrication method according tothe third embodiment.

FIGS. 52( a) through 52(d) are cross-sectional views showing respectiveprocess steps in a third example of the fabrication method according tothe third embodiment.

FIG. 53 is a table showing typical compositions and characteristics ofsilicate phosphors for reference.

FIG. 54 is a table showing experimental data on luminancecharacteristics for a light-emitting semiconductor device using aYAG-based phosphor and a light-emitting semiconductor device using asilicate phosphor.

FIG. 55 is a table showing respective characteristics of samples inwhich ultra-fine-powdery silicon dioxide such as ultra-fine-powderysilica is introduced, as a thixotropy improver, in a silicate phosphorfor a light-emitting semiconductor device.

FIG. 56 is a cross-sectional view showing a structure of alight-emitting semiconductor device including a plurality of blue LEDs.

FIG. 57 is a cross-sectional view showing a structure of alight-emitting system including a large number of blue LEDs and a singleluminescent layer.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiment 1

Hereinafter, a first embodiment of the present invention relating to alight-emitting semiconductor device and a method for suppressing colorunevenness in the light-emitting semiconductor device will be describedwith reference to the drawings.

FIG. 1 is a vertical cross-sectional view showing a first exemplarylight-emitting semiconductor device as a relatively typical example ofthis embodiment. As shown in FIG. 1, the first exemplary light-emittingsemiconductor device is a chip-type light-emitting semiconductor deviceincluding: a substrate 4 (a submount element) functioning as a Zenerdiode; a flip-chip-type blue LED 1 mounted on the substrate 4 to beelectrically connected to the Zener diode in the substrate; and aluminescent layer 3 encapsulating the blue LED 1 and made of a mixtureof yellow/yellowish phosphor particles 2 and a base material 13 (atranslucent resin). The blue LED 1 has a main light-extracting surfacefacing upward as shown in FIG. 1. The luminescent layer 3 is situated sothat blue light emitted from the main light-extracting surface passestherethrough.

FIG. 2 is a vertical cross-sectional view showing a second exemplarylight-emitting semiconductor device according to this embodiment. Asshown in FIG. 2, the second exemplary light-emitting semiconductordevice is a bulletlike light-emitting semiconductor device including: alead frame 5; a cup 6 provided in a mount lead of the lead flame 5; ablue LED 1 mounted in a recess of the cup 6 to be electrically connectedto the lead frame 5 via bonding wires; a luminescent layer 3 formed inthe cup 6 and made of a mixture of yellow/yellowish phosphor particles 2and a base material 13 (a resin); and an encapsulating resin 7 forencapsulating the lead frame 5, the luminescent layer 3 and the bondingwires. The side wall of the recess of the cup 6 functions as areflecting plate that reflects light. The blue LED 1 has a mainlight-extracting surface facing upward as shown in FIG. 2. Theluminescent layer 3 is situated so that blue light emitted from the mainlight-extracting surface passes therethrough.

FIG. 3 is a vertical cross-sectional view showing a third exemplarylight-emitting semiconductor device according to this embodiment. Asshown in FIG. 3, the third exemplary light-emitting semiconductor deviceis a chip-type light-emitting semiconductor device including; anintegrated resin casing 8 having a recess; a blue LED 1 placed in therecess of the casing 8; externally connecting terminals 51 and 52extending from on the bottom of the recess to the outside through therespective sides of the casing 8; bonding wires connecting theexternally connecting terminals 51 and 52 to pad electrodes on the blueLED 1; and a luminescent layer 3 formed in the casing 8 and made of amixture of yellow/yellowish phosphor particles 2 and a resin. The sidewall of the recess of the casing 8 functions as a reflecting plate thatreflects light. The blue LED 1 has a main light-extracting surfacefacing upward as shown in FIG. 3. The luminescent layer 3 is situated sothat blue light emitted from the main light-extracting surface passestherethrough.

In each of the first through third exemplary light-emittingsemiconductor devices shown in FIGS. 1 through 3, the blue LED 1 is anLED which emits light having a main emission peak in the wavelengthrange greater than 430 nm and less than or equal to 500 nm, and theyellow/yellowish phosphor particles 2 is a phosphor which absorbs bluelight emitted by the blue LED 1 to emit a luminescence having a mainemission peak in the wavelength range from 550 nm to 600 nm, bothinclusive. The luminescent layer 3 is a luminescent layer including theyellow/yellowish phosphor particles 2.

A blue-light-emitting device according to the present invention may be adevice selected from among a laser diode, a surface emitting laserdiode, an inorganic electroluminescence device and an organicelectroluminescence device, as well as the blue LED (blue-light-emittingdiode) of this embodiment. However, in terms of increase in the outputand the lifetime of the light-emitting semiconductor device, diodes suchas a light-emitting diode, a laser diode and a surface emitting laserdiode are superior to other devices.

The inventive light-emitting semiconductor device is a light-emittingsemiconductor device configured by combining the blue LED 1 and theluminescent layer 3 containing the yellow/yellowish phosphor particles 2that absorbs blue light emitted by the blue LED 1 to emit a fluorescencehaving an emission peak in the wavelength range from 550 nm to 600 nm,both inclusive. The yellow/yellowish phosphor particles 2 contained inthe luminescent layer 3 is excited by part of the light from the blueLED 1 to produce a fluorescence with a wavelength different from that ofthe light from the blue LED. Accordingly, the color mixture of thefluorescence from the yellow/yellowish phosphor and the light from theblue LED that has been output and does not contribute to excitation ofthe yellow phosphor is produced, thereby allowing emission of white orwhitish light.

Now, the yellow/yellowish phosphor particles 2 are a silicate phosphorcontaining, as a main component, a compound expressed by the followingChemical Formula (1):(Sr_(1−a1−b1−x)Ba_(a1)Ca_(b1)Eu_(x))₂SiO₄  (1)In Chemical Formula (1), the values a1, b1 and x are in the range0≦a1≦0.3, 0≦b1≦0.8 and 0<x<1, respectively).

This silicate phosphor can be in the three kinds of crystal structuresof an orthorhombic system, a monoclinic system and a hexagonal system,as will be described later in detail using experimental data. In theinventive light-emitting semiconductor device, it is sufficient for theyellow/yellowish phosphor to emit a fluorescence having a main emissionpeak in the wavelength range from 550 nm to 600 nm, both inclusive, byabsorbing blue light emitted by the blue LED 1. The crystal structure ofthe silicate phosphor may be any one of the orthorhombic system, themonoclinic system and the hexagonal system.

An experiment done by the present inventors shows that such ayellow/yellowish phosphor is limited to a silicate phosphor containing,as a main component, a compound expressed by the following ChemicalFormula (2):(Sr_(1−a1−b2−x)Ba_(a1)Ca_(b2)Eu_(x))₂SiO₄  (2)In Chemical Formula (2), the values a1, b2 and x are in the ranges0≦a1≦0.3, 0≦b2≦0.6 and 0<x<1, respectively. It is preferable that thevalues a1, b2 and x are in the ranges 0<a1≦0.2, 0<b2≦0.4 and0.005<x<0.1, respectively. It is more preferable that the values a1, b2and x are in the ranges 0<a1≦0.15, 0<b2≦0.3 and 0.01<x<0.05,respectively. It is most preferable that the values a1, b2 and x are inthe ranges 0.01≦a1≦0.1, 0.001≦b2≦0.05 and 0.01<x≦0.02, respectively.

With a composition in which the values a1 and b2 in Chemical Formula (2)are smaller than the ranges mentioned above, the silicate phosphor isliable to have an unstable crystal structure and to include a monocliniccrystal structure, resulting in that the emission characteristics changedepending on the operation temperature. On the other hand, with acomposition in which these values are larger than the ranges, even ifthe crystal structure is an orthorhombic system, the phosphor emitsgreenish light, proving to be not a desirable yellow/yellowish phosphorbut a green/greenish phosphor. Accordingly, even if the phosphor iscombined with a blue LED, a white-light-emitting semiconductor deviceexhibiting an excellent color of light is not achieved. With acomposition in which the amount of addition of Eu x is smaller than therange, the luminous intensity is low. With a composition in which theamount is larger than the range, there arises noticeably not only aproblem that the luminous intensity is low because of concentrationquenching (of luminescence) or self-absorption caused by Eu²⁺ ions butalso a problem of thermal quenching that the luminous intensitydecreases as the ambient temperature increases. The yellow/yellowishphosphor used in the present invention is preferably a silicate phosphorhaving an orthorhombic crystal structure because the silicate phosphoremits the yellow/yellowish light which is excellent in color purity and,therefore, a light-emitting semiconductor device emitting white lightwith excellent color can be provided. Part of Sr, Ba or Ca may besubstituted with Mg or Zn to stabilize the crystal structure of thesilicate phosphor or to enhance the luminous intensity.

To control the color of light emitted by the silicate phosphor, part ofSi may be substituted with Ge. That is to say, the inventivelight-emitting semiconductor device may be a light-emittingsemiconductor device using a yellow/yellowish phosphor containing, as amain component, a compound expressed by the following Chemical Formula(3):(Sr_(1−a1−b1−x)Ba_(a1)Ca_(b1)Eu_(x))₂Si_(1−z)Ge_(z)O₄  (3)where the values a1, b1, x and z are in the ranges 0≦a1≦0.3, 0≦b1≦0.8,0<x<1 and 0≦z<1 (preferably 0≦z≦0.2, respectively). If part of Si issubstituted with Ge, the tendency of the luminous intensity to greatlydecrease is observed. However, if at least the substitution amount of Geis 20 atomic percent (at. %) or more, the main emission peak shifts toshorter wavelengths, thus obtaining emission of greenish light. To holdluminous intensity, the substitution amount of Ge z is preferably assmall as possible and the value z is preferably within the range lowerthan 0.2.

Further, a red phosphor that absorbs light such as blue light emitted bythe blue LED or yellow/yellowish light emitted by the silicate phosphorand has a main emission peak in the red wavelength range greater than600 nm and less than or equal to 660 nm may be additionally used inorder to compensate for the red spectrum of the light emitted by thelight-emitting semiconductor device. A green phosphor that absorbs lightsuch as blue light from the blue LED and has a main emission peak in thegreen wavelength range greater than or equal to 500 nm and lower than550 nm where the luminous efficacy is high may be also additionally usedto enhance the luminous flux.

Materials for such red and green phosphors are not limited to thematerials used in this embodiment. Such a red or green phosphor may be aphosphor made of an inorganic compound or a phosphor made of an organiccompound.

Usages of such red and green phosphors are not limited to the method ofthis embodiment so long as the light-emitting semiconductor devicefurther includes these phosphors (luminescence materials). Thesephosphors may be included in a luminescent layer or may be disposedapart from the luminescent layer so long as each of the phosphorsabsorbs the blue light to emit red or green light and the blue lightpasses through at least the luminescent layer.

Examples of the red phosphor include: phosphors such as a CaS:Eu²⁺phosphor and an SrS:Eu²⁺ phosphor known as a cathodoluminescencematerial or an electroluminescence material; a rare-earth complex or aresin structure including the rare-earth complex disclosed in, forexample, Japanese Laid-Open Publications Nos. 11-246510 and 2000-63682;and an LiEuW₂O₈ phosphor disclosed in, for example, Japanese Laid-OpenPublication No. 2001-267632.

The use of such a red phosphor increases the intensity of red emissionspectrum of the light-emitting semiconductor device, especially awhite-light-emitting semiconductor device, resulting in enhancement ofR9 recited in JIS Z 8726-1990 or color gamut ratio Ga recited in JIS Z8726-1990 for reference, which are known as special color renderingindices representing faithfulness toward red in the field ofillumination, thus allowing the light-emitting semiconductor device toexhibit the color of light in which these indices are high.

Examples of the green phosphor include: an SrGa₂S₄:Eu²⁺ phosphor knownas a cathodoluminescence material or an electroluminescence material;and a silicate phosphor emitting a fluorescence having an emission peakin the wavelength range from 500 nm to 600 nm, both inclusive, andcontaining, as a main component, a compound expressed by the followingChemical Formula (4):(Sr_(1−a3−b3−x)Ba_(a3)Ca_(b3)Eu_(x))₂SiO₄  (4)

In the formula, the values a3, b3 and x are in the ranges 0≦a3≦1, 0≦b3≦1and 0<x<1, respectively.

This (Sr_(1−a3−b3−x)Ba_(a3)Ca_(b3)Eu_(x))₂SiO₄ silicate phosphor is aphosphor different from the above-described silicate phosphor emittingyellow/yellowish light, only in composition and crystal structure.Accordingly, the (Sr_(1−a3−b3−x)Ba_(a3)Ca_(b3)Eu_(x))₂SiO₄ silicatephosphors have similar properties to those of theyellow/yellowish-light-emitting silicate phosphor. Therefore, the use ofgreen-light-emitting silicate phosphor in combination with theyellow-light-emitting silicate phosphor is more preferable in terms ofnot only light-emitting semiconductor device characteristics but alsothe fabrication thereof.

To provide a light-emitting semiconductor device emitting a desiredcolor of light, a plurality of such silicate phosphors having mutuallydifferent compositions and each emitting yellow/yellowish light havingan emission peak in the wavelength range from 550 nm to 600 nm, bothinclusive, may be included in a luminescent layer. The silicatephosphors are phosphors which can emit light covering a large area ofthe yellow/yellowish wavelength range by changing the compositions.Therefore, if a plurality of types of such silicate phosphors arecombined, it is possible to enlarge the color expression range of lightwhich is emitted by the light-emitting semiconductor device, especiallya light-emitting semiconductor device emitting white or whitish light,and determined by adding the colors of blue light from a blue LED and ofyellow/yellowish light from the silicate phosphors.

In terms of color control for light, especially for white or whitishlight, emitted by the light-emitting semiconductor device, it iseffective to include, in a luminescent layer, at least one type ofsilicate phosphor containing a component expressed by Chemical Formula(4) as a main component and differing from a yellow/yellowish phosphorin composition. The silicate phosphor is a phosphor that emits lightwhen excited by blue light with any of the composition ranges for thevalues a3 and b3 and, moreover, that has an emission peak wavelengthvariable in the wide wavelength range of about 505 to 598 nm by changingthe composition of the phosphor. If such a phosphor is additionallyincluded in the luminescent layer, the light-emitting semiconductordevice emits light by adding the colors of blue light emitted by theblue LED and yellow/yellowish light emitted by theyellow/yellowish-light-emitting silicate phosphor emitting light and atleast one of blue-green, green, yellow and orange lights emitted by an(Sr_(1−a3−b3−x)Ba_(a3)Ca_(b3)Eu_(x))₂SiO₄ together, i.e., adding atleast three colors together. Therefore, it is possible to enlarge therange of color control for light emitted by the light-emittingsemiconductor device.

The light-emitting semiconductor device shown in any one of FIGS. 1through 3 may use a substrate containing Cr and a blue LED incombination in order to compensate for the red spectrum of the lightemitted by the light-emitting semiconductor device. Then, blue lightemitted by the blue LED is utilized, thereby allowing the Cr-containingsubstrate capable of conversion into longer wavelengths to emit redlight. In this way, white light with high color rendering performancecan be emitted by color mixing of the blue light from the blue LED, theyellow light from the silicate phosphor and the red light from theCr-containing substrate. That is to say, the present invention isnaturally applicable to any type of light-emitting semiconductor devicessuch as a flip-chip-type, bulletlike or chip-type.

It is sufficient for the silicate phosphor to have a particle size of0.1 μm to 100 μm, both inclusive, in an evaluation of particledistribution with a laser diffraction particle size analyzer (e.g.,LMS-30 produced by Seishin Enterprise Co., Ltd.). However, in terms ofease of the synthesis, availability of a phosphor or formability of aluminescent layer, the particle size preferably ranges from 0.5 μm to 30μm, both inclusive, more preferably from 1 μm to 20 μm, both inclusive,and still more preferably from 2 μm to 10 μm, both inclusive. Withrespect to the particle-size distribution, it is sufficient for thesilicate phosphor to include no particles smaller than 0.01 μm orgreater than 1000 μm. However, for the same purpose as for the particlesize, the particle-size distribution in the silicate phosphor ispreferably close to the normal distribution in the particle-size rangefrom 1 μm to 50 μm, both inclusive.

Such a silicate phosphor can be fabricated by a synthesizing methoddescribed in, for example, the above-mentioned literature (J.Electrochemical Soc. Vol. 115, No. 11(1968) pp. 1181–1184). A method forfabricating a silicate phosphor for the light-emitting semiconductordevice of this embodiment will be described later. Hereinafter,properties of a silicate phosphor will be described in further detail.

FIG. 8 is a graph showing examples of an excitation spectrum (i.e., aspectrum of light for exciting the silicate phosphor) and an emissionspectrum of the orthorhombic silicate phosphor used in this embodiment.FIG. 8 also shows examples of an excitation light spectrum and anemission spectrum of a YAG-based phosphor for comparison in the samegraph.

As shown in FIG. 8, the YAG-based phosphor is a phosphor having threeexcitation-light peaks around 100 nm to 300 nm, 300 nm to 370 nm, and370 nm to 550 nm, respectively, and absorbing light in these narrowwavelength ranges to emit a yellow/yellowish fluorescence having anemission peak in the wavelength range of 550 to 580 nm, i.e., fromyellow-green to yellow. On the other hand, the silicate phosphor is ayellow/yellowish phosphor having an excitation-light peak around 250 to300 nm and absorbing light in the wide wavelength range of 100 to 500 nmto emit a yellow/yellowish fluorescence having an emission peak in thewavelength range of 550 to 600 nm (an example of which is shown in FIG.8), i.e., from yellow-green, yellow to orange. In addition, the silicatephosphor exhibits a low luminous intensity, i.e., 100 to 30% of that ofthe YAG-based phosphor in general, when irradiated with blue light(excitation light) greater than 430 nm and less than or equal to 500 nm.Specifically, when the wavelength of the excitation light is 470 nm, theluminous intensity of the silicate phosphor is half (50%) of theluminous intensity of the YAG-based phosphor.

If the silicate phosphor has a composition in which the values a1, b1,b2 and x are within the respective given ranges shown in Formulas (1)and (2), the excitation light and emission spectra thereof are similarto those shown in FIG. 8.

Now, characteristics of a luminescent layer using the silicateluminescent layer are described.

The exemplary excitation spectrum and emission spectrum shown in FIG. 8show that the silicate phosphor is a yellow/yellowish phosphor having anexcitation light peak around 250 to 300 nm and absorbing light in thewide wavelength range of 100 to 500 nm to emit a yellow/yellowishfluorescence having an emission peak in the wavelength range from 550 to600 nm, i.e., yellow-green, yellow to orange. Accordingly, combinationof the silicate phosphor with a blue LED allows a light-emittingsemiconductor device to emit light by adding the colors of the bluelight from the blue LED and the fluorescence from the yellow/yellowishphosphor together.

Comparison in excitation spectrum between the silicate phosphor and theYAG-based phosphor shown in FIG. 8 as examples indicates that thesilicate phosphor is a phosphor having a relatively high internalquantum efficiency but having a low external quantum efficiency whenirradiated with blue light (excitation light) in the wavelength rangegreater than 430 nm and less than or equal to 500 nm because thereflectance of blue excitation light is high. That is to say, thesilicate phosphor is a phosphor having a so-called low luminous efficacy(external quantum efficiency). For example, in response to theexcitation light of 470 nm, the silicate phosphor emits a fluorescencehaving an intensity only half of the intensity of the light emitted bythe YAG-based phosphor. Therefore, in the case where a uniform color oflight is to be obtained in a white-light-emitting semiconductor devicethat emits white light by adding the colors of the blue light from theblue LED and the yellow light from the yellow/yellowish phosphortogether, if the silicate phosphor is used, a larger amount of aphosphor is used than in the case where the YAG-based phosphor is used,so that the thickness of a luminescent layer is relatively thick. As aresult, the phosphor is less affected by the unevenness created in thesurface of the phosphor, so that variation in thickness of theluminescent layer becomes substantially small, thus obtaining alight-emitting semiconductor device which emits light with small colorunevenness.

If a luminescent layer is formed using the above-described silicatephosphor and a resin, distribution of phosphor particles in theluminescent layer is small, as compared to a known luminescent layerusing a YAG-based phosphor. If a light-emitting semiconductor device isconfigured by using a YAG-based phosphor, phosphor particles are incontact with each other in a luminescent layer, thus arising a problemthat the intensity of the resultant white or whitish light is low, asdescribed above. Such a problem of low intensity because of this reasonis caused not only in the case where a YAG-based phosphor is used butalso in any case where the light-emitting semiconductor device has aluminescent layer in which phosphor particles are in contact with eachother.

On the other hand, if conditions for forming a luminescent layer areselected as those for the light-emitting semiconductor device of thisembodiment, phosphor particles are relatively evenly dispersed in theluminescent layer, thus obtaining a light-emitting semiconductor devicethat emits light with small color unevenness. The reason why the use ofthe silicate phosphor of this embodiment reduces distribution unevennessof the phosphor particles in the luminescent layer has been minutelyinvestigated but has not clarified completely yet. However, thereduction in distribution unevenness of the phosphor particlesdefinitely relates to at least the fact that the difference in specificgravity between the phosphor and the resin is smaller than that betweenthe YAG-based phosphor and the resin.

Hereinafter, the point where the luminescent layer in the light-emittingsemiconductor device of this embodiment has a structure in whichphosphor particles are relatively evenly dispersed throughout a basematerial (a scattering structure) will be described with reference toFIGS. 1 through 3.

In the luminescent layer 3 shown in FIGS. 1 through 3, as has beendescribed above, the yellow/yellowish phosphor particles 2 are aphosphor that absorbs blue light emitted by a blue LED and having a mainemission peak in the wavelength range from 550 nm to 600 nm, bothinclusive, and are also a silicate phosphor. The base material 13 is atranslucent resin such as an epoxy resin, an acrylic resin, a polyimideresin, a urea resin or a silicone resin.

The luminescent layer 3 of the inventive light-emitting semiconductordevice may include a phosphor in addition to the yellow/yellowishphosphor or may include a substance other than a phosphor. Moreover, theluminescent layer 3 may contain a plurality of types of suchyellow/yellowish phosphors.

In the light-emitting semiconductor device of this embodiments so longas the luminescent layer 3 has a structure in which the yellow/yellowishphosphor particles 2 are dispersed in the base material 13 as shown inFIGS. 1 through 3, the size and the shape of the yellow/yellowishphosphor particles 2 in the luminescent layer 3 are not specificallylimited. It has been proved that if silicate phosphor particles are usedas phosphor particles in the luminescent layer, the use of the phosphorparticles having a particle size in the range from 0.5 μm to 30 μm, bothinclusive, allows the luminescent layer to have a structure in which thephosphor particles are dispersed as shown in FIGS. 1 through 3.

The smaller the size of the yellow/yellowish phosphor particles 2 is,the more dispersed the particles in the luminescent layer 3 are.However, since small phosphor particles have a large specific surfacearea (of particles), the ratio of the surface area of the particleswhere a lot of lattice defects are present is high with respect to thevolume of the phosphor particles, so that the luminous intensity oflight from the luminescent layer 3 decreases. On the other hand, if thephosphor particles are large in size, the yellow/yellowish phosphorparticles 2 readily sediment by gravity during the formation of theluminescent layer 3, so that the luminescent layer 3 has a structure inwhich the phosphor particles are less dispersed. In view of this, theparticle size of the yellow/yellowish phosphor is preferably in theabove-described range (i.e., the range from 0.5 μm to 30 μm, bothinclusive), more preferably from 1 μm to 25 μm, both inclusive, andstill more preferably from 3 μm to 20 μm, both inclusive.

A material for the base material 13 is not limited to the materialdescribed in this embodiment so long as the luminescent layer 3 has astructure in which phosphor particles are dispersed as shown in FIGS. 1through 3. The base material 13 may be any material other than a resinso long as the material is translucent. If the base material 13 is aresin, the kind and the absolute specific gravity of the resin is notlimited to this embodiment basically.

In the case where the base material 13 is a resin, as the absolutespecific gravity of the resin comes close to the absolute specificgravity of the yellow/yellowish phosphor particles 2, the phosphorparticles are tend to disperse more and more in the luminescent layer 3.As will be described later, the absolute specific gravity of a resin issmaller than that of the yellow/yellowish phosphor particles 2 ingeneral, the absolute specific gravity of the resin is preferably aslarge as possible within the range lower than the absolute specificgravity of the yellow/yellowish phosphor particles 2.

If the absolute specific gravity of the resin is small, theyellow/yellowish phosphor particles 2 readily sediment by gravity duringthe formation of the luminescent layer 3, so that the phosphor particlesare less readily to disperse in the luminescent layer 3. In view ofthis, the absolute specific gravity of the resin is preferably set inthe range greater than or equal to 0.8 and less than or equal to theabsolute specific gravity of the phosphor particles, more preferably inthe range greater than or equal to 1.0 and less than or equal to theabsolute specific gravity of the phosphor particles, and still morepreferably in the range greater than or equal to 1.5 and less than orequal to the absolute specific gravity of the phosphor particles

According to “Plastic Data Handbook” (edited by Kimimasa ITO andpublished by Kogyo Chosakai Publishing Co. Ltd.) or “NonmetallicMaterial Data Book” (published by Japanese Standards Association), forexample, the absolute specific gravity of an epoxy resin is between 1.0and 2.1, inclusive, the absolute specific gravity of an acrylic resin isbetween 1.0 and 1.4, inclusive, the absolute specific gravity of apolyimide resin is between 1.3 and 1.5, inclusive, the absolute specificgravity of a urea resin is about 1.5 and the absolute specific gravityof a silicone resin is between 1.7 and 2.0, inclusive.

In each of the exemplary light-emitting semiconductor devices shown inFIGS. 1 through 3, the luminescent layer 3 uses a mixture of phosphorparticles and a resin (base material). Alternatively, the luminescentlayer may be made by forming (or sintering) a luminescence material.

—General Fabrication Method—

Concrete examples for fabricating an inventive light-emittingsemiconductor device will be described in detail later. Now, a summaryof a method for fabricating a luminescent layer 3 having a structure inwhich phosphor particles are dispersed and preferred embodiments thereofare described.

A luminescent layer 3 with a structure in which phosphor particles aredispersed can be formed by placing a phosphor paste, in whichyellow/yellowish phosphor particles 2 having an absolute specificgravity within a given range are dispersed in a base material 13 havingan absolute specific gravity within a given range, in a position in alight-emitting semiconductor device through a process such as injectionor application, and then by curing the phosphor paste.

The phosphor paste can be fabricated by weighing and mixing theyellow/yellowish particles 2 and the base material 13 such as a resinsuch that the phosphor paste has a given phosphor concentration. To mixthese materials, various techniques may be used. Examples of thetechniques include mixing using a mortar, stirring using an agitator andmulling using a roller.

In the mixing, the weight percentage of the yellow/yellowish phosphorparticles 2 with respect to the base material 13 (i.e., the phosphorconcentration) is preferably in the range from 10 wt % to 80 wt %, bothinclusive, and more preferably in the range from 20 wt % to 60 wt %,both inclusive. If the phosphor concentration is lower than the ranges,the luminescent layer 3 exhibits weak emission of the yellow/yellowishphosphor so that the light-emitting semiconductor device configured byusing the luminescent layer 3 emits bluer light, resulting in that it isdifficult to obtain white light with excellent color tone. On the otherhand, if the phosphor concentration is higher than the range, theluminescent layer 3 exhibits strong emission of the yellow/yellowishphosphor so that the light-emitting semiconductor device configured byusing the luminescent layer 3 emits yellower light, resulting in that isdifficult to obtain white light with excellent color tone.

In the inventive method for fabricating the luminescent layer, atechnique for curing the phosphor paste is not limited to a specifictechnique. The phosphor paste may be cured by: using a material thatcures by mixing two liquids such that the curing due to mixing of thetwo liquids occurs in the phosphor paste; heating the phosphor pasteusing a thermosetting material; or irradiating the paste with lightusing a photo-curing material. The luminescent layer 3 can be obtainedwith any one of the curing techniques for the phosphor paste.

To form the luminescent layer 3 in which phosphor particles aredispersed, it is preferable to suppress the sedimentation speed of theyellow/yellowish phosphor particles 2 in the base material 13.

Hereinafter, the sedimentation speed of phosphor particles beingsedimenting in a solvent will be described for reference. According toStokes' low, a ball having a radius r (unit: m) and a density ρ₂ isbeing sedimenting in a fluid having a density ρ₁ and a viscositycoefficient η (i.e., viscosity, unit: Pa·s or P (poise)) at asedimentation speed μ (m/s) expressed by the following equation (5):μ={2×r ²×(ρ₂−ρ₁)×g}/(9×η)  (5)In Equation (5), g indicates a gravitational acceleration (unit: m·s⁻²).

Therefore, qualitatively, the sedimentation speed of phosphor particlesbeing sedimenting in a resin as a solvent decreases as the particle sizeof the phosphor particles decreases, the speed also decreases as thedifference in specific gravity between the phosphor particles and theresin narrows, and also decreases as the viscosity of the resinincreases.

From Stokes' law described above, it is possible to reduce thesedimentation speed of the yellow phosphor 3 in the resin by using thefollowing measures 1. through 4.

-   1. Using light phosphor particles having a small absolute specific    gravity-   2. Using a resin having a large absolute specific gravity-   3. Using phosphor particles having a small particle size-   4. Using a resin having a large viscosity    It should be noted that these measures 1. through 4. are subjected    to various constraints such as a constraint on fabrication    processes, on cost or on emission performance of the luminescent    layer.

In the inventive method for fabricating the luminescent layer, each ofthe absolute specific gravity of the yellow/yellowish phosphor particles2 that are limited in main emission peak wavelength range and in elementand the absolute specific gravity of the resin are limited within agiven range. As a preferred embodiment, the particle size of theyellow/yellowish phosphor is further limited to a given range. As a morepreferred embodiment, the kind and composition of the yellow/yellowishphosphor are limited.

First, as the yellow/yellowish phosphor particles 2, used is a phosphorcontaining no Cd (cadmium) and emitting light having a main emissionpeak wavelength in the range from 560 nm to 600 nm, both inclusive,preferably in the range greater than 560 nm and less than or equal to600 nm, and more preferably in the range from 565 nm to 600 nm, bothinclusive, under room temperature. Next, the absolute specific gravityof the yellow/yellowish phosphor particles 2 is limited within the rangefrom 3.0 to 4.65, both inclusive, preferably within the range from 3.0to 4.60, both inclusive, and more preferably within the range greaterthan or equal to 3.0 and less than 4.55. In addition, the absolutespecific gravity of the resin is limited within the range greater thanor equal to 0.8 and less than or equal to the absolute specific gravityof the yellow/yellowish phosphor, preferably within the range greaterthan or equal to 1.0 and less than or equal to the absolute specificgravity of the yellow/yellowish phosphor, and more preferably within therange greater than or equal to 1.5 and less than or equal to theabsolute specific gravity of the yellow/yellowish phosphor.

In this way, the difference in specific gravity between theyellow/yellowish phosphor particles 2 and the resin narrows so that thesedimentation speed of the phosphor particles in the resin decreases, asindicated by Stokes' low in Equation (5), resulting in that theluminescent layer with a structure in which the phosphor particles aredispersed is easily formed.

Especially, examples of yellow/yellowish phosphors containing no Cdinclude a phosphor including, as a base material, a compound containingat least one element selected from the group consisting of Mg, Ca, Sr,Ba, Sc, Y, lanthanoid, Ti, Zr, Hf, V, Nb, Ta, Mo, W, Zn, B, Al, Ga, In,Si, Ge, Sn and P and at least one element selected from the groupconsisting of O, S, Se, F, Cl and Br. These elements are relatively lessnoxious.

The reason why the main peak wavelength of the yellow/yellowish phosphorparticles 2 is limited within the range from 560 nm to 600 nm, bothinclusive, is to obtain white light having a desired color tone. Thereasons why the absolute specific gravity of the resin is limited withinthe above-mentioned ranges and why the phosphor containing no Cd isexclusively used have been already explained.

In the inventive method for fabricating the light-emitting layer of thelight-emitting semiconductor device, the yellow/yellowish phosphorparticles 2 are not specifically limited in kind basically so long asthe yellow/yellowish phosphor particles 2 contain no Cd, exhibits a mainemission peak wavelength in the range from 560 nm to 600 nm, bothinclusive, under room temperature and has an absolute specific gravityin the range from 3.0 to 4.65, both inclusive. The yellow/yellowishphosphor particles 2 may or may not be the silicate phosphor particlesdescribed above.

On the other hand, none of the known light-emitting semiconductordevices uses, as phosphor particles, such light yellow/yellowishphosphor particles 2 which contain no noxious substance, emityellow/yellowish light when excited by blue light, and have an absolutespecific gravity in the ranges described in this embodiment, so that theformation of the luminescent layer for the known devices requires ayellow-light-emitting YAG-based phosphor having a large absolutespecific gravity. The yellow-light-emitting YAG-based phosphor particleshave an absolute specific gravity in the range greater than 4.65 andless than or equal to about 4.98. The spectrum of emission from theyellow-light-emitting YAG-based phosphor particles shifts to longerwavelengths, as the absolute specific gravity thereof increases.Therefore, it is difficult for the known light-emitting devices toobtain excellent characteristics which are obtained in thelight-emitting device of this embodiment.

In a preferred embodiment of the present invention, the particle size ofthe yellow/yellowish phosphor particles 2 is limited within the rangefrom 0.5 μm to 30 μm, both inclusive, preferably within the range from 1μm to 25 μm, both inclusive, and more preferably within the range from 3μm to 20 μm, both inclusive. The reason why the particle size of theyellow/yellowish phosphor particles 2 is limited in this preferredembodiment has been already explained.

In a further preferred embodiment, a silicate phosphor containing, as amain component, a compound expressed by Chemical Formula (1), i.e., acompound expressed by (Sr_(1−a1−b1−x)Ba_(a1)Ca_(b1)Eu_(x))₂SiO₄, is usedas the yellow/yellowish phosphor particles 2. Although the absolutespecific gravity of the silicate phosphor varies to some extentdepending on the composition, the absolute specific gravity of theyellow/yellowish phosphor particles 2 is easily set within the rangefrom 3.0 to 4.65, both inclusive, thereby allowing a luminescent layerhaving a structure in which phosphor particles are dispersed to beformed easily. The specific gravity of the silicate phosphor containinga compound expressed by Chemical Formula (1) as a main componentincreases, as the substitution amount of Ba increases, while decreasingas the substitution amount of Ca increases.

Now, the absolute specific gravities of phosphors are supplementarydescribed. True density measurements of phosphors by constant volumeexpansion with He gas replacement method using a Multivolume Pycnometer1305 produced by Micromeritics Instrument Co. show that the absolutespecific gravities of a YAG-based phosphor((Y_(0.7)Gd_(0.28)Ce_(0.02))₃Al₅O₁₂: main emission peak wavelength of565 nm), a silicate phosphor ((Ba_(0.05)Sr_(0.93)Eu_(0.02))₂SiO₄: mainemission peak wavelength of 575 nm) and a silicate phosphor containing asmaller amount of Si and having a composition different from the formersilicate phosphor ((Ba_(0.24)Sr_(0.74)Eu_(0.02))₂SiO₄: main emissionpeak wavelength of 559 nm), are 4.98, 4.53 and 4.67, respectively(measurement accuracy: +1%). As an example, with respect to the phosphorwhich emits light having a main emission peak around 565 nm, it wasproved that the absolute specific gravity of the silicate phosphor issmaller than that of the YAG-based phosphor by about 10%.

FIG. 48 is a graph showing respective relationships between the absolutespecific gravity and the main emission peak wavelength for a YAG-basedphosphor and a silicate phosphor. As shown in FIG. 48, it is impossibleor, if not impossible, difficult for a YAG-based phosphor to serve as aphosphor that emits yellow/yellowish light having a main emission peakin the wavelength range from 560 nm to 600 nm, both inclusive,especially, in the wavelength range from 565 nm to 600 nm, bothinclusive, and having an absolute specific gravity of 4.65 or less. Onthe other hand, it is easy for a silicate phosphor containing a compoundexpressed by Chemical Formula (1), i.e.,(Sr_(1−a1−b1−x)Ba_(a1)Ca_(b1)Eu_(x))₂SiO₄, to serve as a phosphor thatemits yellow/yellowish light having a main emission peak in thewavelength range from 560 nm to 600 nm, both inclusive, especially, inthe wavelength range from 565 nm to 600 nm, both inclusive, and havingan absolute specific gravity of 4.65 or less.

Now, the viscosities of a resin and a phosphor paste are described. Theinventive method for fabricating a luminescent layer of a light-emittingsemiconductor device is not limited to fabrication methods which will bedescribed later. However, if the viscosities of a resin and a phosphorpaste are too low, phosphor particles sediment by gravity so that astructure in which the phosphor particles disperse in the resin cannotbe achieved, as described above. On the other hand, if the viscosity ofthe resin is too high, there occurs a disadvantage that handling thelight-emitting semiconductor device is cumbersome in a fabricationprocess. In view of these aspects, each of the viscosities of the resinand phosphor is in the range from 0.01 Pa·s to 10 Pa·s, both inclusive,preferably in the range from 0.03 Pa·s to 3 Pa·s, both inclusive, andmore preferably in the range from 0.1 Pa·s to 1 Pa·s, both inclusive.However, the viscosity of a liquid fluid such as a resin or a phosphorpaste varies depending on temperature and pressure, i.e., decreases asthe temperature increases while increasing as the pressure increases.Therefore, it is difficult to define the viscosity simply. The viscosityof the resin or the phosphor paste may be adjusted within the rangesdescribed above by adjusting conditions including pressure andtemperature during the fabrication.

In the inventive method for forming the luminescent layer of thelight-emitting semiconductor device, the phosphor paste may be formed bycuring with ultra-fine particles whose primary particles have an averageparticle size in the range from 1 nm to 100 nm, both inclusive,preferably in the range from 3 nm to 50 nm, both inclusive, included inthe phosphor paste.

As expressed by Equation (5), the sedimentation speed of ultra-fineparticles with an extremely small particle radius is extremely low in aphosphor paste. Accordingly, if such ultra-fine particles are includedin the phosphor paste, the ultra-fine particles, which sedimentextremely slowly, act in such a manner as preventing sedimentation ofthe yellow/yellowish phosphor particles 2. As a result, by adding theultra-fine particles to the phosphor paste, the sedimentation speed ofthe yellow/yellowish phosphor particles 2 in the phosphor pastedecreases, so that the luminescent layer 3 having a structure in whichthe phosphor particles are dispersed in the resin is easily obtained.

Examples of such ultra-fine particles include a silicon dioxide powderknown by the name of Aerosil (Degussa Co., Ltd.: Germany). Materials forultra-fine particles which may be added to the phosphor paste is notlimited to silicon dioxide and may be an ultra-fine particle materialwhose primary particles have an average particle size in the range from1 nm to 100 nm, both inclusive. Instead of silicon dioxide, aluminumoxide, for example, may be used as the ultra-fine particle material.

Ultra-fine particles having a particle size of about 5 nm or less cannotbe measured with the laser diffraction particle size analyzer asmentioned above. Therefore, the particle sizes (diameters) of ultra-fineparticles are actually measured based on the object observed by anelectron microscope observation, thereby defining an average value ofthe particle sizes as the average size of the primary particles.

As described above, with the method for forming the luminescent layer ofthe light-emitting semiconductor device, the luminescent layer 3 havinga structure in which phosphor particles are dispersed is formed. Thelight-emitting semiconductor device including the luminescent layerhaving the structure in which phosphor particles are dispersed achievesremarkable effects with the following actions.

Specifically, such a luminescent layer contains substantially neitherlight absorption factor nor light scattering factor. Therefore, ascompared to the known luminescent layer in which phosphor particles arein contact with each other, for example, phosphor particles are lesslikely to be in contact with each other, or even if these particles arein contact with each other, the contact area is largely reduced, so thatthe luminescent layer has substantially no light absorption andattenuation factor. Accordingly, the luminescent layer exhibits improvedlight transmissivity, thus allowing blue light emitted by a blue LED topass through the luminescent layer without being absorbed and attenuatedor to contribute to excitation of the phosphor. In addition, since theluminescent layer is in the state that the entire surface of thephosphor particles can be irradiated with the blue light, thecross-sectional area of the phosphor particles to be excited increasessubstantially, so that the phosphor particles in the luminescent layeremit light effectively. Part of the blue light which is applied to thephosphor particles but does not contribute to the excitation of thephosphor is reflected off the surfaces of the phosphor particles andthen emitted as blue light to the outside of the luminescent layer.Since blue LEDs of the same type emit blue light with the same output,in a white-light-emitting semiconductor device that obtains white lightby adding the colors of blue light emitted by a blue LED and yellowlight emitted by a yellow phosphor together, the luminescent layerhaving fewer light absorption and attenuation factors and using aphosphor having a high internal quantum efficiency can exhibit a highluminous flux, even if the luminescent layer is made of a phosphormaterial having a low luminous efficacy (external quantum efficiency) inresponse to excitation by the blue light.

If the luminescent layer of the light-emitting semiconductor device ofthis embodiment and the known luminescent layer have the same surfacearea (e.g., the area of the uppermost surface of the luminescent layer 3in the light-emitting semiconductor device shown in FIGS. 1 through 3),the substantial thickness of the luminescent layer having a structure inwhich phosphor particles are dispersed in a resin as in this embodiment(see the luminescent layer 3 shown in FIG. 2, for example) increases, ascompared to the known luminescent layer in which many phosphor particlesare in contact with each other (see the luminescent layer 3 in FIG. 7).Accordingly, in the light-emitting semiconductor device of thisembodiment, even if the surface unevenness of the luminescent layer 3enlarges to some extent, variation in thickness of the luminescent layer3 is less affected by the surface unevenness of the luminescent layer 3,resulting in reducing variation in emission caused by thicknessvariation in the luminescent layer 3.

Embodiment 2

Now, an embodiment of a light-emitting system according to the presentinvention is described with reference to the drawings.

Various kinds of displaying systems using light-emitting semiconductordevices (e.g., LED information display terminals, LED traffic lights,LED stoplights of vehicles and LED directional lights) and various kindsof lighting systems (e.g., LED interior/exterior lights, courtesy LEDlights, LED emergency lights, and LED surface emitting sources) areherein defined broadly as light-emitting systems.

FIGS. 4 through 6 are perspective views respectively showing examples ofa light-emitting system according to a second embodiment of the presentinvention. FIG. 4 is a perspective view schematically showing aconfiguration of a desk-lamp-type lighting system as a first exemplarylight-emitting system of the present invention.

As shown in FIG. 4, the first exemplary light-emitting system includes:a lighting unit in which a large number of light-emitting semiconductordevices 11 according to the present invention as described in the firstembodiment are arranged; and a switch 12 for lighting the light-emittingsemiconductor devices 11. When the switch 12 is turned on, thelight-emitting semiconductor devices 11 are energized to emit light (notshown).

The lighting system shown in FIG. 4 is merely a preferred example of thelight-emitting system, and the inventive light-emitting system is notlimited to this example. The inventive light-emitting system ispreferably configured using the inventive light-emitting semiconductordevices 11 as disclosed in the first embodiment, for example.Alternatively, the inventive light-emitting system may be configured bycombining the white-light-emitting semiconductor device of the firstembodiment with an LED which emits light such as blue, green, yellow orred light. The color of light emitted by the light-emittingsemiconductor devices 11, the size and the number of the devices 11, andthe shape of a light-emitting portion, for example, are not specificallylimited. In addition, the lighting system may be of the laser-emittingsemiconductor type that converges light from the light-emittingsemiconductor devices to emit laser light. In this way, the lightingsystem is not only excellent in field of view as a lighting system butalso capable of improving the intensity of light emitted therefrom.

In the first exemplary lighting system, the color temperature ispreferably in the range from 2000 K to 12000 K, both inclusive, morepreferably in the range from 3000 K to 10000 K, both inclusive, andstill more preferably in the range from 3500 K to 8000 K, bothinclusive. However, the lighting system as the inventive light-emittingsystem is not limited to these color temperature ranges.

FIG. 5 is a perspective view schematically showing a configuration of animage displaying system as a second exemplary light-emitting systemaccording to the present invention.

As shown in FIG. 5, the second exemplary image displaying systemincludes a display unit in which a large number of inventivelight-emitting semiconductor devices 11 as described in the firstembodiment are arranged in matrix. The image displaying system may befreely fabricated in total size and preferably has a width between 1 cmand 10 m, both inclusive, a height between 1 cm and 10 m, bothinclusive, and a depth between 5 mm and 5 m, both inclusive. The numberof the light-emitting semiconductor devices 11 may be selected accordingto the size of the image displaying system.

As is the first exemplary lighting system, the image displaying systemas an example of the light-emitting system is preferably configuredusing the light-emitting semiconductor devices 11 described in the firstembodiment. Instead of the inventive light-emitting semiconductordevices, a device utilizing an LED which emits light such as blue,green, yellow or red light and a luminescent layer in combination may beused, for example. The color of light emitted by the light-emittingsemiconductor devices 11, the size and the number of the devices 11, theshape of a light-emitting portion thereof, and the arrangement of thelight-emitting semiconductor devices 11 are not specifically limited. Inaddition, external shape thereof is not specifically limited.

FIG. 6 is a perspective view schematically showing a configuration of apattern displaying system as a third exemplary light-emitting systemaccording to the present invention.

As shown in FIG. 6, the third exemplary pattern displaying systemincludes a display unit in which the inventive light-emittingsemiconductor devices 11 as described in the first embodiment arearranged such that arbitrary numerals of 0 to 9 can be displayedaccording to emission or non-emission of each pixel.

The pattern displayed by the pattern displaying system is not limited tothe numeral shown in FIG. 6 and may be any pattern representing kanjicharacters, katakana characters, alphabet characters and Greekcharacters. Even if the pattern displaying system displays numerals, thesize and the number of the light-emitting semiconductor devices 11 andthe configuration of pixels are not specifically limited to theconfiguration shown in FIG. 6.

As is the first exemplary lighting system, the pattern displaying systemas an example of the light-emitting system is preferably configuredusing the light-emitting semiconductor devices 11 described in the firstembodiment. Instead of the inventive light-emitting semiconductordevices, a device utilizing an LED which emits light such as blue,green, yellow or red light and a luminescent layer in combination may beused. The color of light emitted by the light-emitting semiconductordevices 11, the size and the number of the devices 11, the shape of alight-emitting portion thereof, and the arrangement of thelight-emitting semiconductor devices 11 are not specifically limited. Inaddition, the external shape thereof is not specifically limited.

The light-emitting systems as shown in FIGS. 4 through 6 has anadvantage that the configuration with a plurality of light-emittingsemiconductor devices 11 using LED chips of only one type allows thelight-emitting semiconductor devices to operate at the same drivingvoltage with the same injected current. In this case, the light-emittingsystem has an advantage that the characteristics of the respectivelight-emitting devices also change substantially in the same manner dueto exogenous factors such as ambient temperature, so that the luminousintensity or color tone of the light-emitting devices less varies withvariation in voltage or temperature. The systems also have an advantagethat the circuit configuration thereof can be made simple.

If light-emitting semiconductor devices having substantially flat pixelsurfaces are used for configuration of a light-emitting system, it ispossible to obtain a light-emitting system whose entire light-emittingsurface is substantially flat, e.g., a displaying system having a flatdisplay surface or a surface-emitting lighting system, thus providing animage displaying system exhibiting excellent image quality or awell-designed lighting system.

In the case where the inventive light-emitting system is a lightingsystem or a displaying system, for example, the use of a light-emittingsemiconductor device having a structure as described in the firstembodiment suppresses color unevenness in the light-emitting system. Thelight-emitting semiconductor device of the first embodiment exhibitssmall color unevenness, resulting in a high production yield and a lowproduction cost. That is to say, if the light-emitting system isconfigured using the light-emitting semiconductor device of the firstembodiment, color unevenness as the light-emitting system is reducedand, in addition, the light-emitting system is fabricated at a low cost.Moreover, since the light-emitting semiconductor device of the firstembodiment exhibits a luminous flux higher than that of a knownlight-emitting semiconductor device using a YAG-based phosphor, theluminous flux of the entire light-emitting system improves.

In this description, various kinds of display systems usinglight-emitting semiconductor devices (e.g., LED information displayterminals, LED traffic lights, LED stoplights of vehicles, and LEDdirectional lights) and various kinds of lighting systems (e.g., LEDinterior/exterior lights, courtesy LED lights, LED emergency lights, andLED surface emitting sources) are broadly defined as light-emittingsystems.

—Embodiment Relating to Method for Fabricating Light-EmittingSemiconductor Device—

(Method for Fabricating Silicate Phosphor)

A method for fabricating a silicate phosphor for use in the inventivelight-emitting semiconductor device is not limited to a fabricationmethod that will be described below. The silicate phosphor is fabricatedby, for example, the following method.

The silicate phosphor can be obtained through the following processes,for example,

-   a first process: weighing and blending of phosphor materials-   a second process: mixing of the phosphor materials-   a third process: firing of the mixed phosphor materials-   a fourth process: subsequent process of the fired material    (including pulverizing, classification, cleaning and drying).    Hereinafter, the respective processes will be described in further    detail.

(First Process: Weighing and Blending of Phosphor Materials)

First, phosphor materials are weighed and blended. As the phosphormaterials, various kinds of powders such as alkaline-earth metalcompounds, silicon compounds and europium compounds may be used.Examples of the alkaline-earth metal compounds include alkaline-earthmetal carbonates (strontium carbonate, barium carbonate and calciumcarbonate), nitrates (strontium nitrate, barium nitrate and calciumnitrate), hydroxides (strontium hydroxide, barium hydroxide and calciumhydroxide), oxides (strontium oxide, barium oxide and calcium oxide),nitrates (strontium nitrate, barium nitrate and calcium nitrate),oxalates (strontium oxalate, barium oxalate and calcium oxalate).Halides (e.g., strontium chloride, barium chloride, calcium chloride,strontium fluoride, barium fluoride, calcium fluoride, strontiumbromide, barium bromide and calcium bromide) may also be used. Examplesof the silicon compounds include oxide such as silicon dioxide andsilicon oxide. However, nonoxide such as silicon nitride may also beused under some conditions. To enhance the reactivity between thephosphor materials, silicon dioxide of an ultra-fine power such as anultra-fine-powdery silica known by the name of “Aerosil” produced byDegussa Co., Ltd. (Germany) is preferably used. Examples of the europiumcompounds include europium oxide, europium fluoride and europiumchloride. As a germanium material for the Ge-containing phosphormentioned above, germanium compound such as germanium oxide may be used.

In the first process, these alkaline-earth metal compound, siliconcompound and europium compound are weighed and blended such that thephosphor has a desired composition of elements such as alkaline-earthmetal, silicon and europium.

To enhance the reactivity between the phosphor materials, a phosphormaterial or a temporary or primary fired phosphor material mixed with aflux may be used. As the flux, various kinds of halides and boroncompounds may be used. Examples of the halides include strontiumfluoride, barium fluoride, calcium fluoride, europium fluoride, ammoniumfluoride, lithium fluoride, sodium fluoride, potassium fluoride,strontium chloride, barium chloride, calcium chloride, europiumchloride, ammonium chloride, lithium chloride, sodium chloride andpotassium chloride. Examples of the boron compounds include boric acid,boric oxide, strontium borate, barium borate and calcium borate. In thecompound used as the flux, the number of moles is between 0.0001 and 1,both inclusive, and normally between 0.001 and 0.3, both inclusive, withrespect to one mole of the phosphor.

(Second Process: Mixing of Phosphor Materials)

Next, the phosphor materials that have been weighed and blended to havespecified mole fractions or weight percentages in the first process aremixed, thereby obtaining a phosphor material mixture. Various techniquesmay be used to mix the phosphor materials. Examples of mixing includemixing using a mortar, mixing using a ball mill, mixing using a V-shapedmixer, mixing using a cross rotary mixer, mixing using a jet mill andmixing using an agitator, all of which are well-known techniques. Drymixing for mixing only the phosphor materials without using any solventor wet mixing for adding the phosphor materials to a solvent such aswater or an organic solvent so that the phosphor materials are spreadand mixed in the solvent may be used as a method for the mixing. Ethanolor methanol may be used as the organic solvent. In the case of the drymixing, the suspension made of the phosphor materials and the solvent isgenerally filtered using, for example, a Buchner filter, to obtain aphosphor material mixture, and then the filtered phosphor materialmixture is dried at a temperature of about 60 to 200° C. for several todozens of hours with, for example, a dryer, thereby obtaining a phosphormaterial mixture.

(Third Process: Firing of Phosphor Material Mixture)

Then, the phosphor material mixture is fired by the following procedure.A heater such as an electric furnace or a gas furnace is used for thefiring. The heater is not specifically limited in type and may be of anytype so long as the phosphor material mixture can be fired at a desiredtemperature in a desired atmosphere for a desired period of time.Examples of the electric furnaces as the heater include a tubularatmospheric furnace, a box-type controlled atmospheric furnace, aconveyor belt furnace, a roller-hearth furnace and a tray pushercontinuous furnace. In general, the phosphor material mixture is put ina firing vessel such as a crucible or a boat with a lid put on thefiring vessel in some cases, and then the phosphor material mixture isheated together with the firing vessel. Alternatively, only the phosphormaterial mixture may be fired. The firing vessel may be made ofplatinum, quartz, alumina, zirconia, magnesia, silicon carbide, siliconnitride, ceramic or carbon, or the firing vessel may be made by mixingthese materials if necessary.

The silicate phosphor can be fabricated so long as the firingtemperature is in the range from 800° C. to 1600° C., both inclusive. Ifthe firing temperature is higher than the temperature range, phosphorparticles are fired or dissolved, so that it is difficult to obtain apowdery silicate phosphor. On the other hand, if the firing temperatureis lower than the temperature range, it is difficult to obtain intensiveemission of light from the phosphor. To obtain a powdery silicatephosphor exhibiting a higher luminous efficacy, the firing temperatureis preferably in the range from 1000° C. to 1500° C., both inclusive,more preferably in the range from 1100° C. to 1450° C., both inclusive,and still more preferably in the range from 1200° C. to 1400° C., bothinclusive.

It is sufficient for the firing time to be in the range from 10 minutesto 1000 hours, both inclusive. However, to increase the efficiency infabrication or to enhance the quality of the phosphor, for example, thefiring time is preferably in the range from 30 minutes to 500 hours,both inclusive, and more preferably in the range from 1 hour to 100hours, both inclusive. It is not specifically limited how many times thefiring process is performed. However, to enhance the efficiency infabricating the phosphor, the firing process is preferably performedless frequently and is most preferably performed only once.

The firing atmosphere may be freely selected from among air, alow-pressure atmosphere, a vacuum atmosphere, an inert-gas atmosphere, anitrogen atmosphere, an oxygen atmosphere, an oxidizing atmosphere and areducing atmosphere. However, since Eu²⁺ ions need to be formed as aluminescent center in the phosphor, it is necessary to perform firing inan atmosphere in which at least Eu²⁺ ions can be formed in the phosphorat the final stage or near final stages of firing. As this atmosphere, areducing atmosphere using a mixed gas of nitrogen and hydrogen or usingcarbon monoxide, especially an ambient using a mixed gas of nitrogen andhydrogen, is preferably used for the purposes of simplifying theapparatus, reducing the cost thereof, and easily handling gases ormaterials for the atmosphere. In the case of the atmosphere using amixed gas of nitrogen and hydrogen, the hydrogen concentration ispreferably in the range from 0.1% to 10%, both inclusive, and morepreferably in the range from 1% to 5%, both inclusive, in terms ofsecuring minimum reduction power and safety of gas. To enhance thereactivity between the mixed phosphor materials, it is preferable thatthe materials are temporary fired in the air, for example, at atemperature between 400° C. and 1400° C. in advance.

(Fourth Process: Subsequent Process of the Fired Substance)

Lastly, the fired substance (phosphor) obtained by the firing process issubjected to a subsequent process, thereby obtaining a silicatephosphor. The subsequent process mainly includes a pulverizing step(which means a step of reducing the substance into a powder), aclassifying step, a cleaning step and a drying step.

In the pulverizing step, the as-fired phosphor obtained by the firing(agglomeration of particles) is reduced into particles. To pulverize thefired substance, various techniques may be used. Examples of thesetechniques include pulverizing with a mortar, pulverizing with a ballmill, pulverizing utilizing a V-shaped mixer, pulverizing utilizing across rotary mixer, pulverizing with a jet mill and pulverizing with acrusher, a motor grinder, a vibrating cup mill, a disk mill, a rotorspeed mill, a cutting mill and a hammer mill. As a method forpulverizing, dry pulverizing for pulverizing the fired substance withoutusing any solvent or wet pulverizing for adding the fired substance to asolvent such as water or an organic solvent to pulverize the firedsubstance within the solvent may be used. As the organic solvent,ethanol or methanol may be used.

In the classifying step, the aggregation of phosphor particles obtainedthrough the pulverizing is changed into an aggregation of particleshaving a given particle-size distribution. As the classification,various techniques may be used. Examples of these techniques includeclassification with a screen and classification utilizing sedimentationof phosphor particles in a solvent such as water or alcohol. In theclassification with a screen, the use of a screen of about 50 to 1000mesh can achieve a silicate phosphor having a particle size within therange (described in the first embodiment) suitable to application to alight-emitting semiconductor device. As a method for classification, dryclassification using neither solvent nor wet classification for addingthe pulverized substance to a solvent such as water or an organicsolvent to classify the pulverized substance together with the solventmay be used. Two or more of these classifying techniques are used insome cases for the purpose of obtaining a sharp particle distribution.

In the cleaning step, a residual flux component contained in the firedsubstance after the firing and fine particles mixed in the productduring the pulverizing or classification step are mainly removed.Various techniques may also be used as the cleaning step. Examples ofthese techniques include cleaning with acid, cleaning with alkali,cleaning with water such as distilled water or pure water, and cleaningwith an organic solvent such as ethanol or methanol. The phosphorparticles after the pulverizing or classification are cleaned by using asolvent appropriately selected according to the type or composition ofthe phosphor materials. The wet pulverizing step or the wet classifyingstep may be used such that the step also serves as the cleaning step.The cleaning step may be omitted depending on the type of the phosphorto be produced.

In the drying step, the aggregation of phosphor particles obtainedthrough the pulverizing, classifying and cleaning steps is heated and alarge or small amount of a solvent such as water or an organic solventcontained in the aggregation is evaporated and dried, thereby obtainingan aggregation of phosphor particles which is a final product or closeto a final product. Various kinds of techniques may be used as thedrying step. Examples of these techniques include drying with athermostatic dryer or a vacuum dryer. If the thermostatic dryer is used,drying is performed at about 60 to 300° C. for about 30 minutes to 100hours. The drying step as well as the cleaning step may be omitted forsome types of a phosphor to be produced.

The pulverizing, classifying, cleaning and drying steps may be flexiblycombined in any sequence and the number of times for performing each ofthe steps may also be determined flexibly according to the type andpurpose of the phosphor.

—Concrete Example of Method for Producing Silicate Phosphor—

Hereinafter, a concrete example of a method for producing a silicatephosphor and effects of a flux will be described with reference toexperimental data.

FIG. 11 is a graph showing a luminous intensity (main emission peakintensity) of a silicate phosphor after primary firing and a luminousintensity (main emission peak intensity) of a silicate phosphor aftersecondary firing, as functions of a primary firing temperature. Theluminous intensity after the primary firing shown in FIG. 11 is data ona primary fired substance obtained by firing phosphor materials, whichhave been blended to have the phosphor composition of(Sr_(0.93)Ba_(0.05)Eu_(0.02))₂SiO₄, at a temperature between roomtemperature and 1400° C. for two hours in a reducing ambient (containinga mixed gas of nitrogen and hydrogen) (primary firing). The luminousintensity after the secondary firing shown in FIG. 11 is data on asecondary fired substance obtained by weighing barium chloride (BaCl₂)and adding the BaCl₂ as a flux to the primary fired substance such thatthe ratio of the (Sr_(0.93)Ba_(0.05)Eu_(0.02))₂SiO₄ silicate phosphor tothe BaCl₂ is 1 mol.:0.1 mol., mixing these substances fully, and thenfiring the mixture at 1400° C. for two hours in a reducing ambient(secondary firing). The luminous intensity of the primary firedsubstance is shown in FIG. 11 for reference. In this manner, thesilicate phosphor can be produced through the procedure of the primaryfiring (which may be omitted), the addition and mixing of the flux andthe secondary firing.

From the X-ray diffraction pattern of the primary fired substance, it isconfirmed that a (Sr_(0.93)Ba_(0.05)Eu_(0.02))₂SiO₄ silicate phosphorhaving an orthorhombic structure is present in the primary firedsubstance fired at a primary firing temperature of 800° C. or more. Itis also confirmed that each of the primary fired substances obtained bybeing fired at primary firing temperatures of 1000° C., 1200° C. and1400° C., respectively, has an orthorhombic crystal structure, i.e., hassubstantially one kind of crystal structure.

From the X-ray diffraction pattern of the secondary fired substance, itis confirmed that every secondary fired substrate is a(Sr_(0.93)Ba_(0.05)Eu_(0.02))₂SiO₄ silicate phosphor having anorthorhombic structure, irrespective of the primary firing temperature.Specifically, FIG. 11 shows that the silicate phosphor can be obtainedthrough the primary firing at 800 to 1400° C. without using a flux, andthat if an additional firing (secondary firing) is performed with a fluxadded and mixed into the primary fired substance fired at roomtemperature (i.e., without primary firing) to 1400° C., a silicatephosphor having a higher luminous intensity (which is about 1.4 to 1.6times of a silicate phosphor obtained with no flux) can be obtained.

—First Concrete Example of Method for Fabricating Light-EmittingSemiconductor Device—

Now, a concrete example of a method for fabricating a light-emittingsemiconductor device according to the present invention is describedwith reference to the drawings. As a first concrete example, a methodfor fabricating a white-light-emitting semiconductor device using atransfer technique and fabrication apparatus therefor are described.FIGS. 12( a) through 12(d) are cross-sectional views showing respectiveprocess steps for fabricating a light-emitting semiconductor device ofthe first concrete example.

First, a blue-light-emitting semiconductor chip 101 as a blue LED shownin FIG. 12( a) is prepared. The blue-light-emitting semiconductor chip101 is, for example, a blue LED using an element such as GaN andexhibiting an emission spectrum having a peak in the wavelength rangefrom 450 nm to 560 nm. The blue-light-emitting semiconductor chip 101includes an anode 101 a and a cathode 101 b on the principal surfacethereof.

In a process step shown in FIG. 12( b), the blue-light-emittingsemiconductor chip 101 is mounted and fixed on a Zener diode 102 by aflip-chip bonding process. In this case, the blue-light-emittingsemiconductor chip 101 and the Zener diode 102 are electricallyconnected to each other. Specifically, the anode 101 a of theblue-light-emitting semiconductor chip 101 and a cathode 102 b of theZener diode are electrically connected to each other, and the cathode101 b of the blue-light-emitting semiconductor chip 101 and an anode 102a of the Zener diode 102 are electrically connected to each other.

Next, in a process step shown in FIG. 12( c), the Zener diode 102 ontowhich the blue-light-emitting semiconductor chip 101 is fixed is mountedand fixed on a substrate 103. In this case, the Zener diode 102 is fixedon the substrate 103 using an adhesive material such as a silver paste.As the adhesive material, other adhesive materials such as solder may beused.

Thereafter, the anode 102 a of the Zener diode 102 is connected to anelectrode terminal 104 provided on the substrate 103. In thisembodiment, to establish this connection, the anode 102 a is connectedto the electrode terminal 104 using a gold wire 105. In this manner, thecathode 101 b of the blue-light-emitting semiconductor chip 101 iselectrically connected to the electrode terminal 104 of the substrate103. The cathode 102 b of the Zener diode 102 may be connected to theelectrode terminal 104 provided on the substrate 103, or the anode 102 aand cathode 102 b of the Zener diode 102 may be connected to respectiveelectrode terminals 104 provided on the substrate 103.

Then, in a process step shown in FIG. 12( d), a resin including phosphorparticles is formed such that light emitted by the blue-light-emittingsemiconductor chip 101 (blue LED) passes through the resin.Specifically, the substrate 103 is placed in a mold 107 and a moldingresin is poured into the mold 107. In general, molding apparatus havinga large number of such molds 107 is used to form a large number ofwhite-light-emitting semiconductor devices at a time. In this case, anepoxy resin 106 in which phosphor particles 108 are dispersed is used asthe molding resin. After that, the white-light-emitting semiconductordevices are taken off from the molds 107. As the epoxy resin, an NTT8506epoxy resin produced by Nitto Denko Co. is used. Subsequently, the epoxyresin is cured.

FIGS. 13( a) and 13(b) are respectively a top view and a cross-sectionalview showing a light-emitting semiconductor device formed by afabrication method of the first concrete example. In FIG. 13( a), theepoxy resin 106 and the phosphor particles 108 are treated astransparent substances. As shown in FIGS. 13( a) and 13(b), obtained isa light-emitting semiconductor device including: a blue-light-emittingsemiconductor chip (blue LED 101) mounted on the substrate 103 via theZener diode 102 and a luminescent layer 109 in which phosphor particles(yellow phosphor particles) 108 are relatively evenly dispersedthroughout the epoxy resin 106.

In this way, the method for fabricating a white-light-emittingsemiconductor device using the transfer technique includes: the processstep of connecting the blue LED 101 as a blue-light-emitting device tothe Zener diode 102 (substrate); and the process step of providing thephosphor particles 108 and the resin 106 such that the light emitted bythe blue LED 101 passes therethrough.

More specifically, the method includes: the step of connecting, on awafer 109 including a plurality of Zener diodes 102 as shown in FIG. 49,blue LEDs as blue-light-emitting devices to the respective Zener diodes;the step of providing a resin including a phosphor such that lightemitted by the blue LEDs passes therethrough; and the process step ofseparating the Zener diodes.

Such a fabrication method allows the fabrication of awhite-light-emitting semiconductor device including a blue LED, a Zenerdiode (a substrate) to which the blue LED is electrically connected anda luminescent layer in which phosphor particles are dispersed in a resinand which is provided such that the light emitted by the blue LED passesthrough the luminescent layer.

In addition, it is also possible to fabricate a white-light-emittingsemiconductor device including no Zener diode and including a blue LEDand a luminescent layer in which phosphor particles are dispersed in aresin and which is provided such that the light emitted by the blue LEDpasses through the luminescent layer.

Examples of materials constituting the blue LED of this concrete exampleinclude a gallium nitride-based compound semiconductor, a zinc selenidesemiconductor and a zinc oxide semiconductor. As a phosphor material,the phosphor used in the first embodiment may be used and, inparticular, a silicate phosphor is preferably selected.

—Second Concrete Example of Method for Fabricating Light-EmittingSemiconductor Device—

Now, a method for fabricating a bulletlike light-emitting semiconductordevice and fabrication apparatus are described as a second concreteexample with reference to the drawings. FIGS. 14( a) through 14(c) arecross-sectional views showing first-half stages of a process forfabricating the light-emitting semiconductor device of the secondconcrete example. FIGS. 15( a) and 15(b) are cross-sectional viewsshowing the latter half of the process for fabricating thelight-emitting semiconductor device of the second concrete example.

First, in a process step shown in FIG. 14( a), the blue LED 101 ismounted and fixed on a frame 110 (a lead frame). The frame 110 includes:a recess 111 for placing the blue-light-emitting semiconductor device101 therein; a terminal 112 continuous to the recess 111; and a terminalnot continuous to the recess 111. These terminals 112 and 113 areconnected to each other by the same metal as these terminals at theopposite side of the recess 111 so as to prevent the terminals fromgoing away from each other in the actual device, but the connectionbetween the terminals will be cut off in a subsequent process step. Theterminal 112 may not be connected to the recess 111. In this case, asshown in FIG. 14( a), the blue LED 101 is placed on the bottom of therecess 111 and is fixed to the terminal 112 using an adhesive materialsuch as a silver paste. Alternatively, other adhesive materials such assolder may be used as the adhesive material.

Thereafter, in a process step shown in FIG. 14( b), an anode and acathode of the blue LED 101 are electrically connected to a terminal 112c and the terminal 113, respectively, via gold wires 114.

Then, in a process step shown in FIG. 14( c), a mixture of phosphorparticles 116 and a resin 115 is poured into the recess 111 of the frame110. In this case, an epoxy resin is used as the resin 115 and thephosphor particles 116 are dispersed in the epoxy resin. The epoxy resinis cured under conditions that the heating temperature is 115° C. andthe heating time is 12 hours, or that the heating temperature is 120° C.and the heating time is five hours. In this manner, a luminescent layer119 in which the phosphor particles 116 are dispersed in the resin 115is formed. In this second concrete example, an epoxy resin produced byFine Polymers Co. is used as the epoxy resin. If a resin material whichdoes not need heat for curing such as an epoxy resin (YL6663 produced byYuka Shell Co. Ltd.) which cures with ultraviolet radiation or a resinmaterial which cures with an curing agent is used as the resin 115 to bepoured into the recess 111, softening of the resin 115 that occursduring the heating is suppressed. Accordingly, it is possible to preventthe sedimentation of the phosphor particles 116 from being promoted dueto the softening of the resin 115 before the resin 115 cures. Therefore,by using a resin material that does not need heat for curing, thephosphor particles 116 more evenly disperse in the resin 115.

Thereafter, in a process step shown in FIG. 15( a), the frame 110 isplaced in a mold 117, while being turned over. Then, a resin 118 formolding is poured into the mold 117. In this case, an epoxy resin isused as the resin 118 for molding. As the resin 118 for molding, anepoxy resin which cures with heat is preferably used in terms of thereliability of a white-light-emitting semiconductor device. However, aresin which does not cure with heat may be used.

Subsequently, the resin is cured, thereby obtaining a bulletlikewhite-light-emitting semiconductor device as shown in FIG. 15( b). Thatis to say, a bulletlike light-emitting semiconductor device includingthe luminescent layer 119 in which the phosphor particles 116 aredispersed in the resin 115 and emitting white light with an excellentcolor tone as described in the first embodiment is obtained.

In this case, it is sufficient for the frame 110 on which the blue LED101 is placed to have a recessed shape in cross-section. Thus, the frame110 includes: a mounting portion 112 a (the bottom face of the recess inthis case) for mounting the blue LED; a side 112 b surrounding themounting portion 112 a; and terminals 112 c and 113 such that aluminescent layer can be formed in a space (the recess 111) made by themounting portion 112 a and the side 112 b. The shape of the recess 111may be any one of a bottomless cylinder, a bottomless polygonal prism, abottomless cone, a bottomless pyramid, a topless or bottomless truncatedcone and a topless or bottomless truncated pyramid.

Thus, the side 112 b is configured to reflect light emitted by the blueLED 101 placed on the mounting portion 112 a (the bottom), so that it ispossible to improve the external light extraction efficiency of theentire light-emitting semiconductor device.

It is preferable that the resin 115 in which the phosphor particles 116are dispersed is supplied to the recess 111 to a level lower than theheight of the side thereof. That is to say, the luminescent layer 119 ispreferably lower than the upper edge of the recess 111. This is commonamong the cases where the shapes of the recess 111 are a cylinder, apolygonal prism, a cone, a pyramid, a truncated cone and truncatedpyramid, respectively. In this manner, in the case where a plurality ofwhite-light-emitting semiconductor devices are provided and lightsemitted by the respective white-light-emitting semiconductor devices areto be utilized, it is possible to solve the problem of crosstalk thatoccurs between adjacent ones of the white-light-emitting semiconductordevices when blue light emitted by one excites phosphor particlesincluded in a resin in the other. In particular, a device which emitswhite light by utilizing blue light from the blue-light-emittingsemiconductor device and yellow light from a phosphor excited by theblue light has a structure with which the blue light is also emitted tothe outside, so that such a problem of crosstalk is serious. However, ifthe luminescent layer 119 is lower than the height of the side 112 b ofthe recess 111, such a problem of crosstalk can be eliminated.

As described above, the method for fabricating the light-emittingsemiconductor device of the second concrete example is a fabricationmethod (or fabrication apparatus) including: a step of (means for)mounting a blue LED 101 on a mounting portion 112 a; and a step of(means for) forming a luminescent layer 119 made of a mixture ofphosphor particles 116 and a resin 115 such that light emitted by theblue LED passes through the luminescent layer 119.

Still more specifically, the fabrication method (or fabricationapparatus) includes: a step of (or means for) providing theblue-light-emitting diode on the mounting portion; a step of (or meansfor) providing a first resin including a phosphor in such a situationthat light emitted by the blue-light-emitting diode passes through theresin; and a step of (or means for) providing a second resin includingno phosphor in such a situation that light emitted by theblue-light-emitting diode passes through the resin. In this case, aresin which does not cure with heat is preferably selected as the firstresin, while a resin which cures with heat is preferably selected as thesecond resin.

As a material constituting the blue LED, a nitride gallium-basedcompound semiconductor, a zinc selenide semiconductor and a zinc oxidesemiconductor may be used. As a phosphor material, the materials used inthe first embodiment may be used and, in particular, a silicate phosphoris preferably selected.

In this concrete example, the epoxy resin is used as the resin 115.Alternatively, any other resins such as a silicone resin may be used.

The anode and cathode of the blue LED are electrically connected to therespective terminals via the gold wires. However, the wires may be madeof any material so long as electric connections are established. Forexample, aluminum wires may used.

—Third Concrete Example of Method for Fabricating Light-EmittingSemiconductor Device—

Now, a method for fabricating a side-view type white-light-emittingsemiconductor device and fabrication apparatus are described as a thirdconcrete example. FIGS. 16( a) through 16(c) are cross-sectional viewsshowing first-half stages of a process for fabricating thelight-emitting semiconductor device of the third concrete example.

First, in a process step shown in FIG. 16( a), the blue LED 101 ismounted and fixed in a casing 120. The casing 120 includes: a base 120for placing the blue LED 101 thereon; a side 121; and externallyconnecting terminals 122 and 123 extending from on the bottom of arecess 128 to the outside through the side 121 of the casing 120. Inthis case, as shown in FIG. 16( a), the blue LED 101 is placed on thebottom of the recess 128 and fixed thereon using an adhesive materialsuch as a silver paste.

Thereafter, in a process step shown in FIG. 16( b), an anode and acathode of the blue LED 101 are electrically connected to the terminals122 and 123 via gold wires 124.

Then, in a process step shown in FIG. 16( c), a mixture of a resin 125and phosphor particles 126 is poured into the recess 128 of the casing120. In this concrete example, an epoxy resin is used as the resin 125and the phosphor particles 126 are dispersed in the epoxy resin. Theepoxy resin is cured under conditions that the heating temperature is115° C. and the heating time is 12 hours, or that the heatingtemperature is 120° C. and the heating time is five hours. In thismanner, a luminescent layer 129 in which the phosphor particles 126 aredispersed in the resin 125 is formed. In this third concrete example, anepoxy resin produced by Fine Polymers Co. is used as the epoxy resin. Ifa resin material which does not need heat for curing such as an epoxyresin (YL6663 produced by Yuka Shell Co. Ltd.) which cures withultraviolet radiation or a resin material which cures with an curingagent is used as the resin 125 to be poured into the recess 128,softening of the resin 125 that occurs during the heating is suppressed.Accordingly, it is possible to prevent the sedimentation of the phosphorparticles 126 from being promoted due to the softening of the resin 125before the resin 125 cures. Therefore, by using a resin material thatdoes not need heat for curing, the phosphor particles 126 disperse moreevenly in the resin 125.

Subsequently, the resin is cured, thereby obtaining a side-view typewhite-light-emitting semiconductor device as shown in FIG. 16( c). Thatis to say, a bulletlike light-emitting semiconductor device includingthe luminescent layer 129 in which the phosphor particles 126 dispersein the resin 125 and emitting white light with an excellent color toneas described in the first embodiment is obtained.

In this case, it is sufficient for the casing 120 on which the blue LED101 is placed to have a recessed shape in cross-section. Thus, thecasing 120 includes a base 120 for mounting the blue LED thereon, a side121 surrounding the recess 128 on the base 120; and the externallyconnecting terminals 122 and 123 such that a luminescent layer can beformed in a space (the recess 128) made by the base 120 and the side121. The shape of the recess 128 may be any one of a bottomlesscylinder, a bottomless polygonal prism, a bottomless cone, a bottomlesspyramid, a topless or bottomless truncated cone and a topless orbottomless truncated pyramid.

Thus, the side 121 is configured so as to serve as a reflecting platefor reflecting light emitted by the blue LED 101 placed on the base 120,so that it is possible to enhance the external light extractionefficiency of the entire light-emitting semiconductor device.

It is preferable that the resin 125 in which the phosphor particles 126are dispersed is supplied to a level lower than the height of the side121 (the side wall of the recess). That is to say, the luminescent layer129 is preferably lower than the upper edge of the recess 128. This iscommon among the cases where the shapes of the recess 128 are acylinder, a polygonal prism, a cone, a pyramid, a truncated cone andtruncated pyramid, respectively. In this manner, in the case where aplurality of white-light-emitting semiconductor devices are provided andlights emitted by the respective light-emitting semiconductor devicesare to be utilized, it is possible to solve the problem of crosstalkthat occurs between one of the white-light-emitting semiconductordevices and an adjacent one of the white-light-emitting semiconductordevices when blue light emitted by the one excites phosphor particlesincluded in a resin in the other. In particular, a device which emitswhite light by utilizing blue light from the blue-light-emittingsemiconductor device and yellow light from a phosphor excited by theblue light has a structure with which the blue light is also emitted tothe outside, so that such a problem of crosstalk is serious. However, ifthe luminescent layer 129 is lower than the height of the side 121, sucha problem of crosstalk can be eliminated.

As described above, the method for fabricating the light-emittingsemiconductor device of the third concrete example is a fabricationmethod including: a step of mounting a blue LED 101 (blue-light-emittingdevice) on a base 120; and a step of forming a luminescent layer 129made of a mixture of phosphor particles 126 and a resin 125 such thatlight emitted by the blue LED passes through the luminescent layer.

More specifically, the fabrication method includes: a step of mountingthe blue-light-emitting device on the base; a step of providing theluminescent layer such that light emitted by the blue-light-emittingdevice passes through the luminescent layer; and a step of providing apermeable resin including no phosphor such that the light emitted by theblue-light-emitting diode passes through the resin. In this case, aresin which does not cure with heat is preferably selected as the resinconstituting the luminescent layer and a resin which cures with heat ispreferably selected as the resin including no phosphor.

As a material constituting the blue LED, a nitride gallium-basedcompound semiconductor, a zinc selenide semiconductor and a zinc oxidesemiconductor may be used. As a phosphor material, the materials used inthe first embodiment may be used and, in particular, a silicate phosphoris preferably selected.

In this concrete example, the epoxy resin is used as the resin 125.Alternatively, any other resins such as a silicone resin may be used.

The anode and cathode of the blue LED are electrically connected to therespective terminals via gold wires. However, the wires may be made ofany material so long as electric connections are established. Forexample, aluminum wires may be used.

In the processes for fabricating the light-emitting semiconductordevices according to the respective concrete examples, the phosphorparticles are preferably dispersed as evenly as possible in the resin.In view of this, concrete examples for dispersing the phosphor particlesevenly throughout the resin in the processes for fabricating alight-emitting semiconductor device will be described below.

—First Concrete Example for Dispersing Phosphor Particles Evenly—

As a first concrete example, a method for applying vibration duringcuring of a resin and an apparatus therefor are described. FIGS. 17( a)and 17(b) are plan views respectively showing two methods for applyingultrasonic vibration in a process for fabricating a light-emittingsemiconductor device. Specifically, as shown in FIG. 17( a), the mold107 is placed in an ultrasonic vibration layer 130 (produced byKAIJODENNKI Co.). While a resin 106 is curing, vibration is applied tothe entire mold 107, so that the phosphor particles 108 are evenlydispersed in the resin 106. Alternatively, vibration may be directlyapplied to the mold 107 by a vibration applying means 131 (such as anultrasonic horn) as shown in FIG. 17( b). For example, even if thephosphor particles 108 sediment in the bottom of the resin 106 in theluminescent layer 109 because of a large difference in specific gravitybetween the resin 106 and the phosphor particles 108, the phosphorparticles 108 and the resin 106 are caused to vibrate by applyingvibration to the mold 107 as shown in FIGS. 17( a) and 17(b), therebyallowing the phosphor particles 108 to disperse evenly in the resin 106as shown in FIG. 21( d).

FIGS. 18( a) and 18(b) are plan views respectively showing two methodsfor applying ultrasonic vibration in the first concrete example of themethod for fabricating the light-emitting semiconductor device (themethod for fabricating the bulletlike light-emitting semiconductordevice) and also showing the states in the application of ultrasonicvibration in the process step shown in FIG. 15( b). Specifically, asshown in FIG. 18( a), the mold 117 is placed in the ultrasonic vibrationlayer 130 (produced by KAIJODENNKI Co.) and vibration is applied to theentire mold 117 while the resin 115 is curing, thereby allowing thephosphor particles 116 to disperse evenly in the resin 115.Alternatively, vibration may be directly applied to the mold 117 by thevibration applying means 131 (such as an ultrasonic horn) as shown inFIG. 18( b). For example, even if the phosphor particles 116 sediment inthe bottom of the resin 115 in the luminescent layer 119 as shown inFIG. 21( a) because of a large difference in specific gravity betweenthe resin 115 and the phosphor particles 116, the phosphor particles 116and the resin 115 are caused to vibrate by applying vibration to themold 117 as shown in FIGS. 18( a) and 18(b), thereby allowing thephosphor particles 116 to disperse evenly in the resin 115 as shown inFIG. 21( c).

In the same manner, in the step shown in FIG. 16( c) in the thirdconcrete example of the method for fabricating the light-emittingsemiconductor device (the method for fabricating the side-view typelight-emitting semiconductor device), the ultrasonic vibration device130 or the vibration applying means 131 may be used. In such a case,even if the phosphor particles 126 sediment in the bottom of the resin125 in the luminescent layer 129 because of a large difference inspecific gravity between the resin 125 and the phosphor particles 126,for example, the phosphor particles 126 are also evenly dispersed in theresin 125 by applying vibration to the phosphor particles 126 and theresin 125.

—Second Concrete Example for Dispersing Phosphor Particles Evenly—

As a second concrete example, a method and apparatus for turning over amold while a resin is curing are described. FIGS. 19( a) and 19(b) arecross-sectional views showing a method for turning over a mold in aprocess for fabricating a light-emitting semiconductor device and alsoshowing the respective states when the mold is turned over. That is tosay, as shown in FIG. 19( a), an inverting means including a rotationshaft 141 and a driving motor (not shown) for rotating the rotationshaft 141 is used and the rotation shaft 141 is installed in the mold107 so that the entire mold 107 is repeatedly turned over between anormal orientation shown in FIG. 19( a) and an inverted orientationshown in FIG. 19( b) while the resin 106 is curing, thereby allowing thephosphor particles 108 to disperse evenly in the resin 106. For example,even if the phosphor particles 108 sediment in the bottom of the resin106 in the luminescent layer 109 as shown in FIG. 21( b) because of alarge difference in specific gravity between the resin 106 and thephosphor particles 108, turning over the mold 107 as shown in FIGS. 19(a) and 19(b) causes the phosphor particles 108 and the resin 106 to moveso that the phosphor particles 108 disperse evenly in the resin 106 asshown in FIG. 21( d).

In this case, as the number of rotating the mold 107 increases, thephosphor particles 108 disperse more evenly in the resin 106. Inaddition, since about 90% of the whole of the resin cures for the firstone hour, the mold 107, i.e., the resin 106, is preferably turned overwithin this first one hour.

FIGS. 20( a) and 20(b) show the respective states in turning over themold in step shown in FIG. 15( a) in the second concrete example of theprocess for fabricating the light-emitting semiconductor device.Specifically, as shown in FIG. 20( a), the inverting means including therotation shaft 141 and a driving motor (not shown) for rotating therotation shaft 141 is used and the rotation shaft 141 is installed inthe mold 117 so that the entire mold 117 is repeatedly turned overbetween a normal orientation shown in FIG. 20( a) and an invertedorientation shown in FIG. 20( b) while the resin 115 is curing, therebyallowing the phosphor particles 116 to disperse evenly in the resin 115.For example, even if the phosphor particles 116 sediment in the bottomof the resin 115 in the luminescent layer 119 as shown in FIG. 21( a)because of a large difference in specific gravity between the resin 115and the phosphor particles 116, turning over the mold 117 as shown inFIGS. 20( a) and 20(b) causes the phosphor particles 116 and the resin115 to move so that the phosphor particles 116 disperse evenly in theresin 115 as shown in FIG. 21( c).

In this case, as the number of rotating the mold 117 increases, thephosphor particles 116 disperse more evenly in the resin 115. Inaddition, since about 90% of the whole of the resin cures for the firstone hour, the mold 117, i.e., the resin 115, is preferably turned overwithin this first one hour.

In the same manner, in the step shown in FIG. 16( c) in the thirdconcrete example of the method for fabricating the light-emittingsemiconductor device (the method for fabricating the side-view typelight-emitting semiconductor device), an inverting means may be used. Insuch a case, even if the phosphor particles 126 sediment in the bottomof the resin 125 in the luminescent layer 129 because of a largedifference in specific gravity between the resin 125 and the phosphorparticles 126, for example, turning over the phosphor particles 126 andthe resin 125 allows the phosphor particles 126 to disperse evenly inthe resin 125.

—Third Concrete Example for Dispersing Phosphor Particles Evenly—

As a third concrete example, a method for performing, a plurality oftimes, a process of filling a recess or a mold with a resin and thencuring the resin is described.

In this concrete example, in the step shown in FIG. 12( d) in the firstconcrete example of the method for fabricating the light-emittingsemiconductor device (the transfer technique), for example, one-third ofthe whole resin 106 including the phosphor particles 108 is poured intothe mold 107 and then cures by heating the resin 106 at 120° C. for fivehours. This process is repeated three times, thereby forming aluminescent layer 109 in the mold 107.

By thus pouring and curing the resin through the process repeated aplurality of times, the phosphor particles 108 disperse relativelyevenly in the resin 106 without sedimenting in the bottom of the resin106 in the luminescent layer 109 as shown in FIG. 21( b).

In the same manner, in the step shown in FIG. 14( c) in the secondconcrete example of the method for fabricating the light-emittingsemiconductor device (the method for fabricating the bulletlikelight-emitting semiconductor device), or in the step shown in FIG. 16(c) in the third concrete example of the method for fabricating thelight-emitting semiconductor device (the method for fabricating theside-view type light-emitting semiconductor device), if the step ofpouring a resin into a mold or a recess and curing the resin isperformed a plurality of times, phosphor particles are relatively evenlydispersed in the resin.

In this case, as the number of steps of pouring and curing the resin inthe mold or recess increases, the phosphor particles disperse moreevenly in the resin. However, since a larger number of the steps requirea longer fabrication time, the number of the steps is preferably five orless, and most preferably about three.

—Fourth Concrete Example for Dispersing Phosphor Particles Evenly—

As a fourth concrete example, a method using a high-viscosity resin informing a luminescent layer will be described.

In this concrete example, in a process for fabricating a light-emittingsemiconductor device (e.g., the step shown in FIG. 17( c)), for example,if the resin 106 including the phosphor particles 108 has a highviscosity, it is possible to prevent sedimentation of the phosphorparticles 108 while the resin 106 is curing. In this concrete example,it is assumed that the resin 106 has a viscosity at which the phosphorparticles 108 do not sediment. The viscosity is preferably in the rangefrom 1 Pa·S to 100 Pa·S, both inclusive.

By thus using the high-viscosity resin, the phosphor particles 108disperse relatively evenly in the resin 106 without sedimenting in thebottom of the resin 106 in the luminescent layer 109 as shown in FIG.21( b).

In the same manner, in the step shown in FIG. 14( c) in the secondconcrete example of the method for fabricating the light-emittingsemiconductor device (the method for fabricating the bulletlikelight-emitting semiconductor device), or the step shown in FIG. 16( c)in the third concrete example of the method for fabricating thelight-emitting semiconductor device (the method for fabricating theside-view type light-emitting semiconductor device), if a high-viscosityresin is used, the phosphor particles disperse relatively evenly in theresin.

—Fifth Concrete Example for Dispersing Phosphor Particles Evenly—

As a fifth concrete example, a method using a resin which does not needheat for curing in forming a luminescent layer will be described.

In this concrete example, in a process for fabricating a light-emittingsemiconductor device (e.g., the step shown in FIG. 12( d)), for example,a resin which cures with ultraviolet radiation (YL6663 produced by YukaShell Co. Ltd.) (which will be hereinafter referred to as anultraviolet-curing resin) is used as the resin 106 including thephosphor particles 108. Alternatively, a resin which cures with a curingagent (hereinafter, referred to as a two-liquid-curing resin) may beused as the resin 106.

As a result, it was found that the use of a resin which cures with heatcauses the phosphor particles 108 to sediment to some extent becausesuch a resin has its viscosity decreased in a certain period beforecuring, whereas the use of the ultraviolet-curing resin ortwo-liquid-curing resin which does not cure with heat allows thephosphor particles 108 to disperse relatively evenly in the resin 106.

In the same manner, in the step shown in FIG. 14( c) in the secondconcrete example of the method for fabricating the light-emittingsemiconductor device (the method for fabricating the bulletlikelight-emitting semiconductor device), or the step shown in FIG. 16( c)in the third concrete example of the method for fabricating thelight-emitting semiconductor device (the method for fabricating theside-view type light-emitting semiconductor device), if anultraviolet-curing resin or a two-liquid-curing resin which does notcure with heat is used, the phosphor particles disperse relativelyevenly in the resin.

If the steps or means for dispersing the phosphor particles more evenlyin the resin are provided as in the first through fifth concreteexamples, the following effects are achieved. That is to say, ascompared to the case where phosphor particles are unevenly distributed,if the phosphor particles are evenly dispersed, especially in a verticaldirection, in the resin, it is possible to outwardly extract blue light(light whose emission spectrum has a peak between 450 nm and 560 nm)emitted by a blue LED, which is otherwise excessively confined in theunevenly distributed phosphor particles. As a result, appropriate whitelight can be obtained.

In addition, a problem that the fluorescence itself emitted by thephosphor particles is confined excessively in the unevenly distributedphosphor particles is eliminated. Therefore, it is possible to extractthe fluorescence to the outside.

Moreover, as compared to the case where phosphor particles are unevenlydispersed in a resin, especially in the case where the phosphorsediments on the substrate 103 as shown in FIGS. 21( a) and 21(c), eventhough the amount of the phosphor is reduced by 10% using the same blueLED, a white-light-emitting semiconductor device with the same colortemperature is achieved, and in addition, the luminance and theintensity can be increased with the same color temperature.

The effects are obtained by using only one of the steps or means in thefirst through fifth concrete examples. However, further synergisticeffects are created by using two or more of the steps or means.

—Concrete Example Relating to Agitation for Luminescent Layer—

FIG. 22 is a cross-sectional view showing a preferred concrete exampleof a phosphor paste discharging apparatus for use in pouring a phosphorpaste containing a silicate phosphor into a cavity in a light-emittingsemiconductor device. In FIG. 22, reference numeral 200 denotes amaterial tank, reference numeral 201 denotes a head, reference sign CAdenotes a cavity in a light-emitting semiconductor device, referencenumeral 204 denotes a pump, reference numeral 205 denotes a dispersionnozzle, reference numeral 206 denotes a phosphor paste, referencenumeral 207 denotes phosphor particles contained in the phosphor paste206, and reference numeral 208 denotes a resin included in the phosphorpaste 206. The head 201 includes: a tank compartment 202 for storing thephosphor paste 206 moved from the material tank 200; a nozzle 203 forejecting the phosphor paste 206 into the cavity CA; and a metal ball Splaced in the tank compartment 202, for example. The phosphor pastestored in the material tank 200 is supplied to the tank compartment 202through the pressurization by the circulating pump 204, and then iscontinuously ejected into the cavity CA through the nozzle 203.

In the phosphor paste 206 stored in the material tank 200 or the tankcompartment 202, the phosphor particles 207 aggregate together with timeand are likely to form a cluster of the phosphor particles 207. Theformation of the cluster of the phosphor particles 207 causes the nozzle203 to clog or the concentration of the phosphor particles 207 in thephosphor paste 206 to be ejected to vary, so that it is difficult todisperse the phosphor particles 207 evenly in the cavity CA in somecases. To avoid the difficulty, in the phosphor paste dischargingapparatus of this example, the phosphor paste 207 stored in the materialtank 200 or the tank compartment 202 is agitated, thereby suppressingthe formation of the cluster of the phosphor particles 207. In theexample shown in FIG. 22, the metal ball S is contained in the materialtank 200 or the tank compartment 202 and the metal ball S is movedwithin the tank by magnetic force, thereby allowing agitation of thephosphor paste 207. In this manner, agglomeration of the phosphor issuppressed in the material tank 200 or the tank compartment 202.

A method for agitating the phosphor paste 207 in the material tank 200or the tank compartment 202 is not limited to the method using the metalball S as shown in FIG. 22, and a lot of other methods may be adopted solong as the variation in concentration distribution in the phosphorpaste 207 within, for example, the tank compartment is suppressed asmuch as possible. For example, vibration may be applied to the tankcompartment 202 or an agitating member may be merely installed on thetank compartment 202. Alternatively, a filter may be placed in thematerial tank 200 so that the phosphor paste 206 is supplied into thematerial tank 200 through the filter, thereby dissolving the cluster.

In addition, the phosphor paste discharging apparatus of this concreteexample is provided with the dispersion nozzle 205 for controlling theflow rate of the phosphor paste 206. Accordingly, when the phosphorpaste 206 passes through the dispersion nozzle 205, the cluster of thephosphor particles 207 in the phosphor paste 206 is separated into smallpieces by a jet stream, thereby disassembling the cluster. If the neckdiameter of the dispersion nozzle 205 is previously set to fit thenozzle diameter of the head 202, the cluster in the phosphor paste 206created in the material tank 200 or in a supply path on the way to thematerial tank 200 is appropriately disassembled, thereby stabilizing theejection from the nozzle 203. By suppressing the agglomeration of thephosphor in the head 202 via the dispersion nozzle 205, the nozzle 203is prevented from clogging and, more over, the phosphor particles 207are easily dispersed evenly in the cavity CA. The dispersion nozzle 205is not necessarily provided and may be adaptively provided according to,for example, the viscosity of the silicate phosphor.

—Electrification of Luminescent Layer—

As described above, a large difference in specific gravity between aYAG-based phosphor and a base material is considered to be one cause ofsedimentation of the phosphor. In addition, the fact that the YAG-basedphosphor is always positively charged is considered another cause. Thatis to say, if a resin as the base material is positively charged, as isthe YAG-based phosphor, the resin and the YAG-based phosphor repel eachother in general, resulting in that the YAG-based phosphor sediments.

On the other hand, considering the fact that a silicate phosphorcontaining a component expressed by the chemical formula(Sr_(1−a1−b1−x)Ba_(a1)Ca_(b1)Eu_(x))₂SiO₄ as a main component does notsediment with respect to the same resin, and the above-describedrelationship between electrification and sedimentation, the silicatephosphor particles are negatively charged, i.e., oppositely charged tothe resin, so that the phosphor and the resin attract each other. As aresult, it is concluded that the silicate phosphor particles dispersethroughout the resin. Examples of such resins to be positively chargedinclude an epoxy resin and a silicone resin.

In view of this, means for facilitating the dispersion of the YAG-basedphosphor particles may be a method for coating the phosphor particleswith an oxide to be negatively charged.

As a method for coating the surfaces of the phosphor particles with anoxide or a fluoride, a suspension of a phosphor paste and a suspensioncontaining coating particles of a needed oxide or fluoride are mixed andagitated, and then subjected to suction filtration. Thereafter, theresultant substance is dried at 125° C. or higher and then is fired at350° C. In order to enhance the adherability between the phosphor andthe oxide or fluoride, a small amount of a resin, an organic silane or aliquid glass may be added.

As a method for applying a coating to the surfaces of the phosphorparticles, a method utilizing the hydrolysis of an organic metalcompound may be used. Then, a coating of SiO₂, which is an oxide readilyto be negatively charged, is applied to the surfaces of the phosphorparticles. In forming an Al₂O₃ film, Al(OC₂H₅)₃ which is aluminumalkoxide, is used for a phosphor so as to be mixed and agitated in analcohol solution, thereby applying Al₂O₃ to the surface of the phosphor.

The present inventors found that by thus attaching or applying, as acoating, a member made of a material which is opposite to the resin inelectrification polarity to the surfaces of the phosphor particles,resin particles surround the phosphor particles to which the memberpositively charged, i.e., oppositely charged to the resin, is attachedor applied as a coating, resulting in suppressing agglomeration of thephosphor particles as well as preventing sedimentation of the phosphorparticles. That is to say, the present inventors found that in eithercase of the YAG-based phosphor particles or the silicate phosphorparticles, if members are selected such that at least the resin, inwhich the phosphor particles are dispersed, and the phosphor particlesare charged oppositely to each other, such pronounced sedimentation ofphosphor particles as observed in the known devices does not occur.

In this case, if the amount of an oxide of fluoride coating which isapplied to the surfaces of the phosphor particles and is to benegatively charged is too small, only a small effect is achieved,whereas if the amount thereof is too large, the coating absorbs emissionof light so that the luminance might decrease. Therefore, the presentinventors have conducted various experiments to find that the amount ofan oxide or fluoride coating which is applied to the surfaces of thephosphor particles and is to be negatively charged is preferably in therange from 0.05% to 2.0% of the phosphor particles in weight.

In this manner, as a further aspect of the present invention, providedis a structure in which an epoxy resin includes a YAG-based phosphor towhich SiO₂ is applied as a coating or attached. That is to say, thepresent invention provides a structure in which an epoxy resin includesa YAG-based phosphor to which an oxide or a fluoride to be negativelycharged is attached or applied as a coating, and also provides a methodfor forming such a structure. More specifically, the present inventionprovides a structure in which a YAG-based phosphor to which an oxide ora fluoride to be negatively charged is attached or applied as a coatingis included in an epoxy resin which is a resin causing the YAG-basedphosphor to be negatively charged, and also provides a method forforming such a structure. Still more specifically, the present inventionprovides a structure in which light emitted by a blue LED passes throughthe epoxy resin and a method for forming such a structure, therebymaking it possible to provide a white/whitish-light-emittingsemiconductor device in which phosphor particles disperse evenly, asshown in FIGS. 21( c) and 21(d).

—Example of Silicate Phosphor for Light-Emitting Semiconductor Device—

Hereinafter, an implementation example of an inventive light-emittingsemiconductor device will be described.

(Example of Procedure for Forming Silicate Phosphor)

First, silicate phosphor particles having a composition enough to emityellow/yellowish light was formed. Powders of barium carbonate (BaCO₃),strontium carbonate (SrCO₃), europium oxide (Eu₂O₃) or silicon dioxide(SiO₂) were used as materials for the phosphor. Calcium chloride (CaCl₂)was used as a flux. Each of the materials has a purity of 99.9% or moreand a particle size in the range from 10 nm to 5 μm, both inclusive. Toeliminate errors in weighing caused by an adsorption gas, variation inweight of each of the materials before and after heating performed atabout 900° C. in the air was previously examined and grasped.

After 9.9 g, 138.0 g, 2.6 g, 30.7 g and 1.7 g of powdery bariumcarbonate, strontium carbonate, europium oxide, silicon dioxide andcalcium chloride, respectively, had been weighed with an electricbalance, these powders were fully mixed with an automatic mortar,thereby obtaining a mixed phosphor material powder. Thereafter, analumina boat was charged with the mixed phosphor material powder andthen was placed in a given position within a tubular atmospheric furnaceusing an alumina as a core tube, and then firing was performed. Thefiring was performed at a heating temperature of 1400° C. in an ambientcontaining 5% hydrogen and 95% nitrogen for two hours.

After it was confirmed that the interior of the core tube was cooled toroom temperature, the fired substance (silicate phosphor) was taken outand subjected to subsequent processes such as pulverizing, cleaning,classification and drying. In this manner, a silicate phosphor having anorthorhombic structure and emitting yellow/yellowish light was obtained.

Hereinafter, results of pre-evaluation performed on characteristics ofthe obtained silicate phosphor will be described. In this evaluation,the composition of the silicate phosphor, the excitation spectrum andemission spectrum of the silicate phosphor, the reflecting spectrum ofblue excitation light, and the emission spectrum of a phosphor excitedby blue light, were evaluated using a crystal component of the silicatephosphor particles obtained by an X-ray diffraction, a particledistribution and particle size of the silicate phosphor particlesmeasured with a laser diffraction particle size analyzer, and an ICPspectroscopy.

FIG. 23 is an X-ray diffraction pattern showing a result of an X-raydiffraction analysis performed on a silicate phosphor and also showing arelationship between diffraction angle and X-ray diffraction intensity.The X-ray diffraction pattern shown in FIG. 23 is the same as an X-raydiffraction pattern of an orthorhombic Sr₂SiO₂ compound, which will bedescribed later (see FIG. 27( b)). This shows that the silicate phosphorof the implementation example is a (Si, Ba, Eu)₂SiO₄ phosphor with onekind of crystal structure having an orthorhombic structure.

FIG. 24 is a graph showing a particle-size distribution in the silicatephosphor observed by an X-ray diffraction. As shown in FIG. 24, particlesizes of the silicate phosphor particles of the implementation exampleare distributed within the range from about 3 μm to 30 μm, bothinclusive, and the phosphor is made of a group of phosphor particleshaving a particle size of 11.5 μm. As a result of an electron microscopeobservation, it is shown that one particle of the silicate phosphor is acluster made of several round primary particles. Although slight surfaceroughness is observed, the surfaces of the primary particles arerelatively smooth.

Next, the composition of the silicate phosphor was evaluated using anICP spectroscopy. As a result, it was found that the composition of thesilicate phosphor is(Ca_(0.015)Sr_(0.92)Ba_(0.05)Eu_(0.015))Si_(0.99)O_(x), whichsubstantially coincides with the composition of an as-charged silicatephosphor.

Then, an excitation spectrum and an emission spectrum of the silicatephosphor were evaluated. A result of this evaluation is already shownwith reference to FIG. 8. For comparison, an excitation spectrum and anemission spectrum of a YAG-based phosphor are also shown in FIG. 8. FIG.8 shows that the silicate phosphor of this example is a yellow/yellowishphosphor having an excitation-light peak around 250 to 300 nm andabsorbing light in the wide wavelength range from 100 to 500 nm toproduce an emission at a peak wavelength of 569 nm. The yellow/yellowishlight emitted by the silicate phosphor has a chromatically point (x, y)of (0.484, 0.506) in a CIE chromaticity diagram.

FIG. 25 is a graph showing a result of evaluation performed on emissionof the silicate phosphor through integration using an integratingsphere. In this case, the silicate phosphor obtained by theabove-described process is irradiated with blue light for excitationwith a wavelength of 470 nm so that the reflecting spectrum of the blueexcitation light and the emission spectrum of a phosphor excited by theblue light were evaluated. The blue light with a wavelength of 470 nmwas obtained by sending light from a Xe lamp through a monochromator.For comparison, FIG. 25 also shows the reflecting spectrum and emissionspectrum of a YAG-based phosphor. In FIG. 25, the emission peak at 470nm is caused by the excitation light (blue light). FIG. 25 shows thatthe silicate phosphor has a tendency to reflect at least three times asmuch blue light, as the YAG-based phosphor, and that as compared to theYAG-based phosphor, the silicate phosphor exhibits a low luminousintensity, which is about half of the luminous intensity of theYAG-based phosphor, when excited by the blue light.

—Various Characteristics of Silicate Phosphor—

Hereinafter, the characteristics of the silicate phosphor formed by theabove-described procedure will be described in detail. FIG. 53 is atable showing typical compositions and characteristics of silicatephosphors for reference. FIG. 53 indicates compositions quantitativelyevaluated by an ICP spectroscopy basically or compositions which can beestimated from the results of the quantitative evaluation.

Firstly, a relationship between the composition and the crystalstructure of a silicate phosphor is described. The followingdescriptions are about a silicate phosphor obtained by setting the Euconcentration (which is defined as Eu/(Sr+Ba+Ca+Eu)) at a typical valueof 2 at. % (i.e., Eu concentration=0.02) and by being fired at 1400° C.in a reducing atmosphere for two hours.

As has been described above, the silicate phosphor can be in at leastthree kinds of crystal structures such as an orthorhombic system, amonoclinic system and a hexagonal system depending on its composition.These kinds of crystal structures are described with reference to FIGS.26( a) through 30(b).

FIGS. 26( a) and 26(b) are X-ray diffraction patterns respectivelyshowing a (Sr_(0.98)Eu_(0.02))₂SiO₄ phosphor containing neither Ca norBa and a publicly known monoclinic Sr₂SiO₄ compound. FIGS. 27( a) and27(b) are X-ray diffraction patterns respectively showing a(Sr_(0.93)Ba_(0.05)Eu_(0.02))₂SiO₄ phosphor containing no Ca andcontaining 5 at. % Ba in terms of substitution amount and a publiclyknown orthorhombic Sr₂SiO₄ compound. FIGS. 28( a) and 28(b) are X-raydiffraction patterns respectively showing a (Ba_(0.98)Eu_(0.02))₂SiO₄phosphor containing neither Ca nor Sr and a publicly known orthorhombicBa₂SiO₄ compound. FIGS. 29( a) and 29(b) are X-ray diffraction patternsrespectively showing a (Ca_(0.38)Ba_(0.60)Eu_(0.02))₂SiO₄ phosphorcontaining 38 at. % Ca and 60 at. % Ba and a publicly known hexagonalBa_(0.3)Ca_(0.7)SiO₄ compound. FIGS. 30( a) and 30(b) are X-raydiffraction patterns respectively showing a (Ca_(0.98)Eu_(0.02))₂SiO₄phosphor containing neither Sr nor Ba and a publicly known monoclinicCa₂SiO₄ compound.

These X-ray diffraction patterns show data measured at room temperatureand atmospheric pressure. FIGS. 26( b), 27(b), 28(b), 29(b) and 30(b)show data on compounds publicly known by JCPDS (Joint Committee onPowder Diffraction Standards) card and indicate respective numbers ofthe compounds. Comparisons of the X-ray diffraction patterns in FIGS.26( a) through 30(a) with the corresponding X-ray diffraction patternsin FIGS. 26( b) through 30(b) show that the crystal structures of thephosphors formed in this example are a monoclinic system, anorthorhombic system, an orthorhombic system, a hexagonal system and amonoclinic system, respectively.

The relationship between the compositions and the main crystalstructures of silicate phosphors are shown in FIG. 53. The (Si,Ba)₂SiO₄:Eu²⁺ phosphors and the (Ca, Sr)₂SiO₄:Eu²⁺ phosphors can be inthe crystal structures of a monoclinic system and an orthorhombicsystem. The (Ca, Ba)₂SiO₄:Eu²⁺ phosphors can be in the crystal structureof an orthorhombic system, a hexagonal system and a monoclinic system.The (Sr, Ba, Ca)₂SiO₄:Eu²⁺ phosphors in which the substitution amount ofSr (=Sr/(Sr+Ba+Ca+Eu)) is 50 at. % or more have an orthorhombicstructure.

With respect to the crystal structure, (Sr_(1−a1−x)Ba_(a1)Eu_(x))₂SiO₄phosphors are worthy of special remark. A pure (Sr_(1−x)Eu_(x))₂SiO₄phosphor containing no Ba has a monoclinic structure at least in the Euconcentration range 0≦x≦0.1. However, if the(Sr_(1−a1−x)Ba_(a1)Eu_(x))₂SiO₄ phosphor contains about 1 at. % or moreBa in terms of substitution amount (=Ba/(Sr+Ba+Ca+Eu)), the phosphor hasan orthorhombic structure at least in the Eu concentration range 0≦x≦0.3(see FIG. 53).

FIGS. 31( a) and 31(b) are X-ray diffraction patterns respectivelyshowing a (Sr_(0.84)Ba_(0.14)Eu_(0.02))₂(Si_(0.8)Ge_(0.2))O₄ phosphor inwhich part of Si is substituted with Ge and a publicly knownorthorhombic Sr₂SiO₄ compound. FIGS. 31( a) and 31(b) are used forreference and show patterns that coincide with each other. Accordingly,it is shown that the (Sr_(0.84)Ba_(0.14)Eu_(0.02))₂(Si_(0.8)Ge_(0.2))O₄phosphor in which part of Si is substituted with Ge has an orthorhombicstructure. Although experimental data is omitted, the(Sr_(0.84)Ba_(0.14)Eu_(0.02))₂(Si_(0.8)Ge_(0.2))O₄ phosphor in whichpart of Si is substituted with Ge has an orthorhombic structure in theentire range in which the Ge substitution amount (=Ge/(Si+Ge)) is 0 to100 at. %.

Next, a relationship between the composition and the emissioncharacteristics of each silicate phosphor in this example is described.The following descriptions are also about silicate phosphors eachobtained by setting the Eu concentration (which is defined asEu/(Sr+Ba+Ca+Eu)) at a typical value of 2 at. % and by being fired at1400° C. in a reducing atmosphere for two hours.

FIG. 32 is a graph showing emission spectra of(Sr_(0.98−a3)Ba_(a3)Eu_(0.02))₂SiO₄ phosphors having different Basubstitution amounts (a3). FIG. 33 is a graph showing emission spectraof (Ca_(b3)Sr_(0.98−b3)Ba_(0.05)Eu_(0.02))₂SiO₄ phosphors containing 5at. % Ba in terms of substitution amount and having different Casubstitution amounts (b3). FIG. 34 is a graph showing emission spectraof (Ca_(b3)Ba_(0.98−b3)Eu_(0.02))₂SiO₄ phosphors having different Casubstitution amounts (b3). FIGS. 32 through 34 are graphs for use inreference.

FIG. 35 is a graph showing emission spectra of(Ca_(0.19)Sr_(0.55)Ba_(0.24)Eu_(0.02))₂SiO₄ phosphor in which the Casubstitution amount (b3) is 19 at. % and the Ba substitution amount (a3)is 24 at. %. Data shown in FIG. 35 is obtained by combining the resultsmeasured under the excitation with ultraviolet radiation having awavelength of 254 nm together, for convenience in experiments.

Comparison of the emission spectra under excitation by blue light, andexcitation by ultraviolet radiation with a wavelength of 254 nm showsthat the emission spectra are similar to each other, though thecomparison was made for only part of the samples.

Although the excitation-light spectra of the respective silicatephosphors are not shown, it is confirmed through visual observationsthat the inventive (Sr_(1−a3−b3−x)Ba_(a3)Ca_(b3)Eu_(x))₂SiO₄ silicatephosphor is a phosphor which can emit blue-green, green, yellow toorange light when excited by at least blue light with a main emissionpeak wavelength of 470 nm in the entire composition range in varyingdegrees and which has a main emission peak wavelength in the range from505 nm to 598 nm.

Among the (Sr_(1−a3−b3−x)Ba_(a3)Ca_(b3)Eu_(x))₂SiO₄ silicate phosphors,a relatively high luminous efficacy is observed especially in thosewhich contain a large proportion of Si.

FIG. 36 is a graph showing a dependence of the main emission peakwavelength on the Ba substitution amount (a3) in a(Sr_(0.98−a3)Ba_(a3)Eu_(0.02))₂SiO₄ phosphor (silicate phosphor). FIG.53 also shows a relationship between the Ba substitution amount (a3) andthe main emission peak wavelength in the(Sr_(0.98−a3)Ba_(a3)Eu_(0.02))₂SiO₄ phosphor. As can be seen from thesedrawings, in the range of the Ba substitution amount in the silicatephosphor greater than or equal to 0 at. % and less than 0.3 at. %, thesilicate phosphor has a main emission peak wavelength around 535 to 545nm and emits green light. On the other hand, in the Ba substitutionamount range from 0.3 at. % to 24 at. %, both inclusive, the silicatephosphor has a main emission peak wavelength in the yellow range from550 nm to 600 nm, both inclusive, and emits yellow/yellowish light.Considering experimental errors, effects of an impurity and thecharacteristics under special conditions such as high-temperatureenvironment, for example, it is conjectured that the silicate phosphorin which the Ba substitution amount is in the range from 0 at. % toabout 30 at. %, both inclusive, can emit yellow/yellowish light.

FIG. 37 is a graph showing a dependence of the main emission peakwavelength of on the Ca substitution amount (b3) in a(Ca_(b3)Sr_(0.98−b3)Ba_(0.05)Eu_(0.02))₂SiO₄ phosphor (a silicatephosphor). As shown in FIG. 37, in the range of the Ca substitutionamount between 0 at. % and 57 at. %, both inclusive, the silicatephosphor has a main emission peak wavelength in the yellow range from550 nm to 600 nm, both inclusive. The silicate phosphor in which the Casubstitution amount is in the range equal to or less than about 70 at. %emits yellow/yellowish light. Considering, for example, experimentalerrors, it is conjectured that the silicate phosphor((Ca_(b3)Sr_(0.93−b3)Ba_(0.05)Eu_(0.02))₂SiO₄ phosphor) in which the Casubstitution amount is in the range from 0 at. % to about 80 at. %, bothinclusive, can emit yellow/yellowish light.

FIG. 38 is a graph showing a dependence of the main emission peakwavelength on the Ca substitution amount (b3) in a(Ca_(b3)Ba_(0.98−b3)Eu_(0.02))₂SiO₄ phosphor (a silicate phosphor). Asshown in FIG. 38, in the entire composition range of the(Ca_(b3)Ba_(0.98−b3)Eu_(0.02))₂SiO₄ phosphor, the(Ca_(b3)Ba_(0.98−b3)Eu_(0.02))₂SiO₄ phosphor has a main emission peakwavelength in the green range greater than or equal to 500 nm and lessthan 550 nm and emits not yellow/yellowish light but green/greenishlight.

As can be seen from the emission spectrum shown in FIG. 35, a(Ca_(0.19)Sr_(0.55)Ba_(0.24)Eu_(0.02))₂SiO₄ phosphor has a main emissionpeak wavelength in the yellow range from 50 nm to 600 nm, bothinclusive, and emits yellow/yellowish light.

In summary, yellow/yellowish light is obtained from a silicate phosphorwhich is limited in composition range, i.e., the Ba substitution amount(a3) is in the range 0≦a3≦0.3 and the Ca substitution amount (b3) is inthe range 0≦b3≦0.8. It is preferable that the Ba substitution amount(a3) is in the range 0<a3≦0.2 and the Ca substitution amount (b3) is inthe range 0<b3≦0.7. As can be seen from FIG. 53, every silicate phosphorwhose composition is in these ranges has an orthorhombic structure.

FIG. 39 is a graph showing an emission spectrum of a(Sr_(0.84)Ba_(0.14)Eu_(0.02))₂(Si_(0.8)Ge_(0.2))O₄ phosphor in whichpart of Si is substituted with Ge, for reference. As shown in FIG. 39,this phosphor can also emit light when excited by blue light and theluminous intensity thereof decreases largely as the Ge substitutionamount (=Ge/(Si+Ge)) increases. However, at least in the Ge substitutionamount range from 20 at. % to 100 at. %, the light emitted by thephosphor is yellow-green (main emission peak wavelength: about 550 nm).

Next, a relationship among the Eu²⁺-luminescent-center concentration(=Eu/(Sr+Ba+Ca+Eu): equal to Eu concentration), the crystal structureand the emission characteristics of each silicate phosphor is described.The following descriptions are about silicate phosphors each having thecomposition of (Sr_(1−x)Eu_(x))₂SiO₄ or(Sr_(0.95−x)Ba_(0.05)Eu_(x))₂SiO₄ and obtained by being fired at 1400°C. in a reducing atmosphere for two hours.

FIG. 40 is a graph showing emission spectra of (Sr_(1−x)Eu_(x))₂SiO₄phosphors having mutually different Eu concentrations (x) for reference.FIG. 41 is a graph showing emission spectra of(Sr_(0.95−x)Ba_(0.05)Eu_(x))₂SiO₄ phosphors for reference. Data shown inFIGS. 40 and 41 are each obtained as a result of a measurement under theexcitation by ultraviolet radiation with a wavelength of 254 nm. Crystalstructures of these phosphors are briefly described as follows. As aresult of evaluation on a X-ray diffraction pattern, a(Sr_(1−x)Eu_(x))₂SiO₄ phosphor in which at least the Eu concentration(x) is in the range 0≦x≦0.1 has a monoclinic structure. A(Sr_(0.95−x)Ba_(0.05)Eu_(x))₂SiO₄ phosphor in which at least the Euconcentration (x) is in the range 0≦x≦0.3 and a (Sr_(1−x)Eu_(x))₂SiO₄phosphor in which at least the Eu concentration (x) is 0.3 haveorthorhombic structures.

FIG. 42 is a graph showing respective dependences of the main emissionpeak wavelengths on the Eu concentrations in a (Sr_(1−x)Eu_(x))₂SiO₄phosphor and a (Sr_(0.95−x)Ba_(0.05)Eu_(x))₂SiO₄ phosphor. As shown inFIG. 42, the crystal structures of the silicate phosphors descried aboveare correlated with colors of lights emitted therefrom. Specifically, a(Sr_(1−x)Eu_(x))₂SiO₄ phosphor which has a monoclinic structure and inwhich at least the Eu concentration (x) is in the range 0.001≦x≦0.1 hasa main emission peak wavelength in the green range greater than or equalto 500 nm and less than 550 nm. On the other hand, a(Sr_(0.95−x)Ba_(0.05)Eu_(x))₂SiO₄ phosphor which has an orthorhombicstructure and in which at least the Eu concentration (x) is in the range0.001≦x≦0.3 and a (Sr_(1−x)Eu_(x))₂SiO₄ phosphor which has anorthorhombic structure and in which at least the Eu concentration (x) is0.3 have main emission peak wavelengths in the yellow range from 550 nmto 600 nm, both inclusive.

As can be seen from the experimental data, only orthorhombic silicatephosphors which are limited in composition as described above emityellow/yellowish light observed when excited by ultraviolet radiationwith a wavelength of 254 nm or the blue light.

From the foregoing experimental results, proper ranges of the respectiveelements of a silicate phosphor for achieving effects of the presentinvention are as follows.

Ba

Yellow light ranges between 550 nm and 600 nm, both inclusive, inwavelength. Therefore, from FIG. 32, it is understood that yellowwavelengths are obtained from the compound on condition that the Basubstitution amount has a mole fraction between 0.0 and 0.3. From theexperimental results on a compound in which the Ba substitution amounthas a mole fraction b of 0.24 and a compound in which the mole fractionb is 0.43, it is easily conjectured that yellow wavelengths are alsoobtained using a compound in which the mole fraction b is 0.3, thoughthe experimental result thereof is not shown in FIG. 32.

FIG. 36 shows that yellow wavelengths are obtained from the compound oncondition that the Ba substitution amount is between 0 and 30 at. %.From the experimental results of a compound in which the Ba substitutionamount is 24 at. % and a compound in which the Ba substitution amount is43 at. %, it is readily conjectured that yellow wavelengths are alsoobtained from a compound in which the Ba substitution amount is 30 at.%, though the experimental result thereof is not shown in FIG. 36.

Ca

FIG. 33 shows that an optimum condition for obtaining yellow wavelengthsfrom the compound is that the Ca substitution amount has a mole fractionbetween 0.0 and 0.6. From the experimental results of a compound inwhich the Ca substitution amount has a mole fraction of 0.57 and acompound in which the mole fraction is 0.76, it is conjectured thatyellow wavelengths are also obtained from a compound in which the Casubstitution amount has a mole fraction of 0.7, though the experimentalresult thereof is not shown. In addition, from the experimental results,it is conjectured that the emission peak wavelength of a compound inwhich the Ca substitution amount has a mole fraction of 0.8 deviatesfrom the yellow wavelength range. However, considering experimentalerrors included, the condition for obtaining yellow wavelengths from acompound is considered that the Ca substitution amount has a molefraction between 0.0 and 0.8, including 0.7 and 0.8, in addition to thecondition that the Ca substitution amount has a mole fraction between0.0 and 0.6.

FIG. 37 shows that yellow wavelengths are obtained from the compound oncondition that the Ca substitution amount is between 0 and 80 at. %.From the experimental results in the cases where the Ca substitutionamount is 57 at. % and where the Ca substitution amount is 76 at. %, itis conjectured that yellow wavelengths are also obtained in the casewhere the Ca substitution amount is 70 at. %, though the experimentalresult thereof is not shown. The experimental result shows that the mainemission peak wavelength of a compound in which the Ca substitutionamount is 80 at. % deviates from the yellow wavelength range, but the Casubstitution amount of 80 at. % is also included in the optimumconditions, considering experimental errors.

Sr

FIGS. 34 and 38 show that compounds containing no Sr do not emit yellowlight.

Crystal Structure

FIG. 42 shows that a compound having a monoclinic structure does notachieve a yellow wavelength, irrespective of the Eu substitution amount,whereas a compound having an orthorhombic structure achieves a yellowwavelength, irrespective of the Eu substitution amount.

Eu

FIG. 43 shows that a composition having an orthorhombic structureachieves a yellow wavelength, irrespective of the Eu substitutionamount, and that the Eu substitution amount is preferably 10% or less inconsideration of the emission peak height.

FIG. 43 shows a relationship between the Eu concentration and a luminousintensity (main emission peak intensity (height)). The luminousintensities of the (Sr_(1−x)Eu_(x))₂SiO₄ and(Sr_(0.95−x)Ba_(0.05)Eu_(x))₂SiO₄ phosphors exhibit the same tendency toincrease as the Eu concentration increases, and to gradually decreaseafter the luminous intensity has reached the maximum value around the Euconcentration of 1 to 1.5 at. %. For such aspects as the luminousintensity, the shape of an emission spectrum and the chromaticity, theEu concentration (mole fraction x) is preferably in the range0.005<x≦0.1, more preferably in the range 0.01<x≦0.05 and mostpreferably in the range 0.01<x≦0.02, as shown in FIGS. 43, 41 and 53.

Now, a Sr₂SiO₄:Eu²⁺, silicate phosphor and a (BaSr)₂SiO₄:Eu²⁺ phosphorrecited as green phosphors in Japanese Laid-Open Publication No.2001-143869 and herein mentioned as prior art are described As has beendescribed using the experimental data, so far as experiments done by thepresent inventors are concerned, the Sr₂SiO₄:Eu²⁺ silicate phosphor is aphosphor which can be in two crystal structures of an orthorhombicsystem and a monoclinic system, depending on an impurity such as Bacontained in trace amounts. At least as far as the amount of addition ofEu²⁺ luminescent centers practically used (=the number of Eu atoms/(thenumber of Sr atoms+the number of Eu atoms: x) is in the range0.01<x<0.05 at room temperature and atmospheric pressure, anorthorhombic Sr₂SiO₄:Eu²⁺ (α′-Sr₂SiO₄:Eu²⁺) is a yellow/yellowishphosphor emitting yellow/yellowish light at a main emission peak aroundthe wavelength between 560 and 575 nm, and a monoclinic Sr₂SiO₄:Eu²⁺(β′-Sr₂SiO₄:Eu²⁺) is a green phosphor emitting green light at a mainemission peak around the wavelength between 535 and 545 nm (see FIGS. 42and 53).

Experiments done by the present inventors also show that the mainemission peak wavelength hardly varies if the Eu mole fraction(=Eu/(Sr+Eu) atomic percent) is in the practical range from 0.001 to0.3, both inclusive (i.e., the range from 0.1 at. % to 30 at. %, bothinclusive), especially in the range from 0.003 to 0.03, both inclusive.Therefore, the Sr₂SiO₄:Eu²⁺ green phosphor recited in Japanese Laid-OpenPublication No. 2001-143869 can be considered a monoclinic Sr₂SiO₄:Eu²⁺phosphor.

It is already publicly known that the crystal structure of a Sr₂SiO₄compound can be in an orthorhombic system or a monoclinic systemdepending on a small amount of Ba contained (e.g., in G. PIEPER et al.,Journal of the American Ceramic Society, Vol. 55, No. 12 (1972) pp.619–622). It is also known that the crystal structure of a monoclinicSr₂SiO₄ compound undergoes a reversible change into an orthorhombicsystem at about 383 K (see, for example, M. Catti et al., Acta Cryst.,B39 (1983) pp. 674–679).

A compound in which the content percentage of Ba impurity atoms withrespect to Sr atoms (Ba/(Sr+Ba) atomic percent, hereinafter referred toas Ba content) is about 1% or more, i.e., a compound having a Ba contenthigher than that of a (Sr_(0.99)Ba_(0.01))₂SiO₄:Eu²⁺, has anorthorhombic structure and has a main emission peak wavelength varyingfrom around 575 nm to around 505 nm as the Ba content increases (seeFIGS. 32, 36 and 53). As can be seen from FIGS. 32, 36 and 53, among thesilicate phosphors containing a component expressed by the chemicalformula (Sr_(0.98−a3)Ba_(a3)Eu_(0.02))₂SiO₄ a as a main component (wherethe value a3 is in the range 0≦a3≦0.98), a(Sr_(0.98−a3)Ba_(a3)Eu_(0.02))₂SiO₄ silicate phosphor having thecomposition range 0.01≦a3≦0.3 is a yellow/yellowish phosphor having amain emission peak in the wavelength range from 550 nm to 600 nm, bothinclusive, and a silicate phosphor having the composition range0.3<a3≦0.98 is a green/greenish phosphor having a main emission peak inthe wavelength range greater than or equal to 505 nm and less than 550nm, considering measurement errors in the experiments, for example.

Another experiment done by the present inventors shows that the mainemission peak wavelength hardly varies so long as the Eu concentrationhas a practical mole fraction in terms of luminous intensity, i.e., isin the range from 0.001 to 0.3, both inclusive, especially in the rangefrom 0.003 to 0.03, both inclusive. Therefore, the (BaSr)₂SiO₄:Eu²⁺green phosphor recited in Japanese Laid-Open Publication No. 2001-143869can be considered a (Sr_(1−a3−x)Ba_(a3)Eu_(x))₂SiO₄ silicate phosphor(where the value x is in the range 0.001≦x≦0.3) having at least thecomposition range 0.3<a3≦0.98.

Lastly, comparison results in emission characteristics between asilicate phosphor ((Ca_(0.015)Sr_(0.92)Ba_(0.05)Eu_(0.015))₂SiO₄phosphor which has an orthorhombic structure and emits yellow/yellowishlight and in which the Eu concentration is optimized, and a YAG-basedphosphor ((Y_(0.7)Gd_(0.28)Ce_(0.02))₃Al₅O₁₂), are described.

—Luminance Characteristic Comparison Between YAG-Based Phosphor andSilicate Phosphor—

First, the difference in luminance characteristic between alight-emitting semiconductor device using a YAG-based phosphor and alight-emitting semiconductor device using a silicate phosphor isdescribed.

FIG. 54 is a table showing experimental data on luminancecharacteristics for a light-emitting semiconductor device using aYAG-based phosphor and a light-emitting semiconductor device using asilicate phosphor. FIG. 54 refers to the type, weight percentage,luminance, total luminous flux, total radiant flux and chromaticity foreach main phosphor material.

FIG. 54 shows that weight percentages enough to obtain yellow/yellowishlight are slighter in the YAG-based phosphors (samples D and E) than inthe silicate phosphors (the other samples). Specifically, in obtaininglight around the chromaticity (0.35, 0.35), the YAG-based phosphors havephosphor weight percentages of 7.4% (sample D) and 9.8% (sample E),whereas samples A, B and C using silicate phosphors have phosphor weightpercentage of about 50% and exhibit no decreased luminous flux. Thesefacts show that in the YAG-based phosphors, the conversion efficiencyfor converting blue light between 410 nm and 530 nm, both inclusive,emitted by a blue LED into yellow/yellowish light between 550 nm and 600nm, both inclusive, is lower than that in the silicate phosphors. Thatis to say, the conversion efficiency is high in the YAG-based phosphors,and only a small amount of phosphor is used in a luminescent layer toobtain yellow light with an appropriate intensity. As a result, it isconsidered that phosphor particles are liable to disperse unevenly in abase material. On the other hand, in the case of the silicate phosphors,a larger amount of phosphor is used in a light-emitting semiconductordevice, and a substantially thick luminescent layer is formed in thelight-emitting semiconductor device. As a result, it is considered thatthe thixotropy of the phosphor paste improves (i.e., the thixotropyindex falls within a proper range), so that phosphor particles are lessliable to disperse unevenly in a base material for the luminescent layeras well as the phosphor particles are kept to be dispersed and scatteredevenly, resulting in that the occurrence of color unevenness issuppressed.

Then, weight percentages of the silicate phosphors are varied usingsamples F, G, H, I, J and K so as to measure effects on variations inluminance, chromaticity, total luminous flux and total radiant flux.

FIG. 44 is a graph showing a relationship between the phosphorconcentration and the luminance. FIG. 45 is a graph showing arelationship between the phosphor concentration and the total luminousflux. FIG. 46 is a graph showing a relationship between the phosphorconcentration and the total radiant flux. FIG. 47 is a graph showing arelationship between the phosphor concentration and the chromaticity(value x). FIGS. 44 through 47 are graphs showing respective resultsbased on data shown in FIG. 54. FIGS. 44 through 47 show measurementvalues in the cases where the phosphor is 30 wt. %, 40 wt. % and 50 wt.%, respectively. As the weight percentage of the phosphor increases, theluminance, total luminous flux and total radiant flux tend to decrease.On the other hand, as the weight percentage of the phosphor increases,the chromaticity (value x) increases so that the luminescent tends to beyellower. Therefore, it can be said that the weight percentage of thephosphor is preferably 30% or higher. In addition, the weight percentageof the phosphor is more preferably in the range from 30% to 50%, bothinclusive.

—Addition of Thixotropy Improver—

Now, described are effects obtained in the case where anultra-fine-powdery silicon dioxide such as an ultra-fine-powdery silica(product name: “Aerosil” produced by Degussa Co., Ltd. (Germany)) isintroduced, as a thixotropy improver (which is herein an agent having afunction of improving a thixotropy index), in a silicate phosphor for alight-emitting semiconductor device.

FIG. 55 is a table showing respective characteristics of samples inwhich ultra-fine-powdery silicon dioxide such as ultra-fine-powderysilica is introduced, as a thixotropy improver, in a silicate phosphorfor a light-emitting semiconductor device.

Data shown in FIG. 55 is obtained as results of experiments using threesamples of: sample 1 containing only a 30 wt. % silicate phosphor;sample 2 containing an about 30 wt. % silicate phosphor and having anAerosil concentration of 0.57%; and sample 3 containing an about 30 wt.% silicate phosphor and having an Aerosil concentration of 1.11%. FIG.55 shows results on the luminance, total luminous flux and total radiantflux for the respective samples in the case where the chromaticity (x,y) is around (0.3, 0.3). From the comparison between sample 2 and sample3, it is shown that as the Aerosil concentration increases, theluminance improves as well as the luminous flux and the radiant fluxincrease.

The respective standard deviations of the luminance, chromaticity (valuex), total luminous flux, total radiant flux for each of the samples areshown. The standard deviations of all the items for sample 3 are thesmallest among the three samples, and therefore sample 3 is the mostreliable. Although the values of the chromaticities differ to somedegree among the samples and therefore the data therefor is used only asa reference, the luminance, luminous flux and radiant flux for sample 1are high but the standard deviation of the chromaticity for sample 1 isthe highest among the three samples, and thus it can be understood thatthe reliability of sample 1 is low.

From the foregoing, it is conjectured that as the amount of Aerosil tobe added increases, the luminance, luminous flux and radiant fluxincrease as well as the reliability improves. This is because thethixotropy of the silicate phosphor paste is enhanced with Aerosil andtherefore the viscosity of the phosphor paste is appropriately set.Specifically, while the silicate phosphor paste is introduced in thelight-emitting semiconductor device, potting is smoothly performed withthe viscosity kept appropriate, so that the silicate phosphor particlesdisperse relatively evenly in the phosphor paste. After the paste isplaced in a cavity in the light-emitting device, the viscosity thereofshifts to higher levels than during the potting, thus keeping the statein which the silicate phosphor particles do not sediment but disperserelatively evenly in a base material. In this manner, the colorunevenness as observed in YAG-based phosphors is suppressed, resultingin improvement in the luminance, luminous flux and reliability.

Embodiment 3

In this embodiment, a method for forming a thin luminescent layer isdescribed. According to this embodiment, yellow/yellowish phosphorparticles which are expressed by the chemical formula(Sr_(1−a1−b1−x)Ba_(a1)Ca_(b1)Eu_(x))₂SiO₄ where the values a1, b1 and xare in the ranges 0≦a1≦0.3, 0≦b1≦0.8 (more preferably 0≦b1≦0.6) and0<x<1, respectively, and which emit light having an emission peak in thewavelength range from 550 nm to 600 nm, both inclusive, are denselydistributed near a light-emitting diode, so that a luminescent layer ismade thin, i.e., the thickness through which light passes is reduced,thereby reducing attenuation of light. For example, this embodiment isfor a method for fabricating a light-emitting semiconductor device inwhich a luminescent layer has a substantial thickness in the range from50 μm to 1000 μm, both inclusive, where a light extraction surface of ablue-light-emitting device is located.

Hereinafter, examples of the fabrication method will be described.

—First Exemplary Fabrication Method—

FIGS. 50( a) through 50(c) are cross-sectional views showing respectiveprocess steps in a first exemplary fabrication method according to thisembodiment.

First, in a process step shown in FIG. 50( a), a substrate 303 and alight-emitting diode 302 (e.g., a blue LED) mounted on the substrate 303are placed within a cavity of a mold 301. Then, a first phosphor paste307 including, as main components, a base material 310 of a translucentresin and phosphor particles 311 containing a yellow/yellowish phosphoris poured into the mold 301 from a vessel 305. In this case, thephosphor paste 307 is poured to a level higher than the upper surface ofthe light-emitting diode 302. The light-emitting diode 302 has a mainlight extracting surface, which is a surface facing upward in FIG. 50(a).

Next, in a process step shown in FIG. 50( b), a second phosphor paste308 containing phosphor particles 311 at a lower concentration than thatin the first phosphor paste 307 is poured into the mold 301 from avessel 306.

Then, in a process step shown in FIG. 50( c), the resin is cured so thatthe phosphor particles 311 are densely distributed in a portion of thebase material 310 near the light-emitting diode 302, especially in aportion located over the main light extracting surface, whereas thephosphor particles 311 are sparsely dispersed in the base material 320located apart from the light-emitting diode 302. Thereafter, theresultant light-emitting semiconductor device is taken out from the mold301.

In this manner, it is possible to fabricate a white-light-emittingsemiconductor device in which the phosphor particles 311 are denselydistributed in a portion of the base material 310 located at least overthe main light extracting surface of the light-emitting diode 302 andwhich exhibits small color unevenness. If such a light-emittingsemiconductor device is installed in any of the light-emitting systemsshown in FIGS. 4 through 6, a white-light-emitting system in which colorunevenness is suppressed can be fabricated.

—Second Exemplary Fabrication Method—

FIGS. 51( a) through 51(c) are cross-sectional views showing respectiveprocess steps in a second exemplary fabrication method according to thisembodiment.

First, in a process step shown in FIG. 51( a), a substrate 403 and alight-emitting diode 402 (e.g., a blue LED) mounted on the substrate 403are placed within a cavity of a mold 401. Then, phosphor particles 411containing a yellow/yellowish phosphor are sprinkled on the vicinity ofthe light-emitting diode 402 in the mold 401, especially on a main lightextracting surface of the diode 402. The main light extracting surfaceof the light-emitting diode 402 is a surface facing upward as shown inFIG. 51( a).

Next, in a process step shown in FIG. 51( b), a phosphor paste 408including, as main components, a base material 410 of a translucentresin and a small amount of phosphor particles 411 containing ayellow/yellowish phosphor is poured into the mold 401 from a vessel 405.

Then, in a process step shown in FIG. 51( c), the resin is cured so thatthe phosphor particles 411 are densely distributed in a portion of thebase material 410 near the light-emitting diode 402, especially in aportion located over the main light extracting surface, whereas thephosphor particles 411 are sparsely dispersed in a portion of the basematerial 410 apart from the light-emitting diode 402. Thereafter, theresultant light-emitting semiconductor device is taken out from the mold401.

In this manner, it is possible to fabricate a light-emittingsemiconductor device in which the phosphor particles 411 are denselydistributed in a portion of the base material 410 located at least overthe main light extracting surface of the light-emitting diode 402 andwhich exhibits small color unevenness. If such a light-emittingsemiconductor device is installed in any of the light-emitting systemsshown in FIGS. 4 through 6, a light-emitting system with small colorunevenness can be fabricated.

—Third Exemplary Fabrication Method—

FIGS. 52( a) through 52(d) are cross-sectional views showing respectiveprocess steps in a third exemplary fabrication method according to thisembodiment.

First, in a process step shown in FIG. 52( a), a substrate 503 and alight-emitting diode 502 (e.g., a blue LED) mounted on the substrate 503are placed within a cavity of a mold 501. Then, a suspension 507containing, as main components, a volatile solvent 510 and phosphorparticles 511 including a yellow/yellowish phosphor is poured into themold 501 from a vessel 505. In this case, the suspension 507 is pouredto a level higher than the upper surface of the light-emitting diode502. The light-emitting diode 502 has a main light extracting surface,which is a surface facing upward as shown in FIG. 52( a).

Next, in a process step shown in FIG. 52( b), the volatile solvent 510in the suspension 507 is evaporated by heating or pressure reduction.

Then, in a process step shown in FIG. 52( c), a phosphor paste 508including, as main components, a base material 512 of a translucentresin and a small amount of phosphor particles 511 containing ayellow/yellowish phosphor is poured into the mold 501 from a vessel 506.

Then, in a process step shown in FIG. 50( d), the resin is cured so thatthe phosphor particles 511 are densely distributed in a portion of thebase material 512 near the light-emitting diode 502, especially in aportion located over the main light extracting surface, whereas thephosphor particles 511 are sparsely dispersed in a portion of the basematerial 512 located apart from the light-emitting diode 502.Thereafter, the resultant light-emitting semiconductor device is takenout from the mold 501.

In this manner, it is possible to fabricate a white-light-emittingsemiconductor device in which the phosphor particles 511 are denselydistributed in a portion of the base material 512 located at least overthe main light extracting surface of the light-emitting diode 502 andwhich exhibits small color unevenness. If such a light-emittingsemiconductor device is installed in any of the light-emitting systemsshown in FIGS. 4 through 6, a white-light-emitting system with smallcolor unevenness can be fabricated.

(Fourth Exemplary Fabrication Method)

The difference in specific gravity between the phosphor and a basematerial is considered a cause of sedimentation of a YAG-based phosphor.The fact that the YAG-based phosphor is positively charged is alsoconsidered another cause of the sedimentation. Specifically, if a resinas the base material is positively charged, as is the YAG-basedphosphor, the resin and the YAG-based phosphor repel each other ingeneral, so that the YAG-based phosphor sediments.

On the other hand, considering the fact that silicate phosphor particlescontaining a compound expressed by(Sr_(1−a1−b1−x)Ba_(a1)Ca_(b1)Eu_(x))₂SiO₄ as a main component does notsediment in the same resin, and considering the relationship between theelectrification and the sedimentation, the attracting relationshipbetween the positively charged resin and the negatively charged silicatephosphor particles presumably contributes to substantially evendistribution of the silicate phosphor particles in the resin. Examplesof such resins to be positively charged include an epoxy resin and asilicone resin.

In view of this, means for sedimenting the silicate phosphor may be amethod for coating the phosphor particles with, for example, an oxide tobe positively charged.

Examples of the method for coating the surface of the phosphor with anoxide or a fluoride include the following methods. First, suspensionsincluding phosphor particles and coating particles of a needed oxide orfluoride are mixed and agitated, and then subjected to suctionfiltration. Thereafter, the substance remaining after the filtration isdried at 125° C. or higher and then is fired at 350° C. In order toimprove the adherability between the phosphor particles and the oxide orfluoride, a small amount of a resin, an organic silane or a liquid glassmay be added.

To apply the coating, a method utilizing the hydrolysis of an organicmetal compound may also be used. For example, if an Al₂O₃ film is to beformed, Al(OC₂H₅)₃ which is aluminum alkoxide, is used for a phosphorand is mixed and agitated in an alcohol solution, thereby applying Al₂O₃to the surface of the phosphor.

If the amount of positively charged oxide or fluoride coating which isapplied to the surfaces of the phosphor particles is too small, only asmall effect is achieved, whereas if the amount thereof is too large,the coating absorbs light emitted so that the luminance decreases andtherefore such a large amount of the coating is also unfavorable. As aresult of experiments, it is found that the amount is preferably in therange from 0.05% to 2.0% of the phosphor particles in weight.

In this manner, it is possible to fabricate a white-light-emittingsemiconductor device in which phosphor particles are densely distributedin a portion of a base material located at least over a main lightextracting surface of a light-emitting diode and which exhibits smallcolor unevenness. If such a light-emitting semiconductor device isinstalled in any of the light-emitting systems shown in FIGS. 4 through6, a white-light-emitting system with small the color unevenness can befabricated.

According to the fabrication method of the third embodiment, it ispossible to obtain a light-emitting semiconductor device in which aluminescent layer has a substantial thickness in the range from 50 μm to1000 μm, both inclusive, where a light extraction surface of ablue-light-emitting device is located.

Other Embodiments

In the foregoing embodiments, a single blue LED is provided as ablue-light-emitting device in a light-emitting semiconductor device.However, the inventive light-emitting semiconductor device is notlimited to these embodiments.

FIG. 56 is a cross-sectional view showing a structure of alight-emitting semiconductor device including a plurality of blue LEDs.As shown in FIG. 56, the light-emitting semiconductor device includes: aplurality of blue LEDs 601 arranged on a substrate 604; and aluminescent layer 603 covering respective main light extracting surfaces(upper surfaces in the state shown in FIG. 56) of the blue LEDs 601. Theluminescent layer 603 includes: phosphor particles 602 of ayellow/yellowish phosphor having the composition described in each ofthe embodiments; and a resin 613 serving as a base material in which thephosphor particles 602 are dispersed. The resin 613 may be made of anyof the materials described in the embodiments. A Zener diode may bemounted on the substrate 604.

With this structure, it is possible to improve the luminous intensity ofthe white-light-emitting semiconductor device, or adjust the luminousintensity depending on the number of the mounted blue LEDs 601.

In the embodiment of the light-emitting system, the example in which alarge number of light-emitting semiconductor devices each including oneblue LED and one luminescent layer is described. However, the inventivelight-emitting system is not limited to the embodiment.

FIG. 57 is a cross-sectional view showing a structure of alight-emitting system including a large number of blue LEDs and a singleluminescent layer. As shown in FIG. 57, the light-emitting systemincludes: a large number of blue LEDs 651 (blue-light-emitting devices)supported by a supporter 654; and a single luminescent layer 653provided on the entire surfaces of the blue LEDs 651. The luminescentlayer 653 includes: two glass substrates; a resin 663 as a base materialfilled in a gap between the two glass substrates; and phosphor particles652 dispersed in the resin 663. The periphery of the luminescent layer653 is supported by the supporter 654. The phosphor particles 652 areconstituted by a yellow/yellowish phosphor having the compositiondescribed in the embodiments. The resin 613 may be made any of thematerials described in the embodiments.

In the structure shown in FIG. 57, the single luminescent layer 653 issufficient for the large number of blue LEDs 651. Therefore, thefabrication cost is reduced and the fabrication process is simplified.

INDUSTRIAL APPLICABILITY

With respect to the inventive light-emitting semiconductor device,various kinds of displaying systems (e.g., LED information displayterminals, LED traffic lights, LED stoplights of vehicles, and LEDdirectional lights) and various kinds of lighting systems (e.g., LEDinterior/exterior lights, courtesy LED lights, LED emergency lights, andLED surface emitting sources) are defined broadly as light-emittingsystems. In particular, the inventive light-emitting semiconductordevice is suitable for systems utilizing white light.

1. A light-emitting semiconductor device comprising: ablue-light-emitting diode which emits blue light, theblue-light-emitting diode having a light extraction surface; and aluminescent layer which includes a yellow/yellowish phosphor thatabsorbs blue light emitted by the blue-light-emitting diode to emit ayellow/yellowish fluorescence, wherein the luminescent layer is exposedon the blue-light-emitting diode, the yellow/yellowish phosphor containsa compound expressed by the chemical formula:(Sr_(1−a1−b2−x)Ba_(a1)Ca_(b2)Eu_(x))₂SiO₄ (where 0≦a1≦0.3, 0≦b2≦0.6,0.005 <x<0.1), the yellow/yellowish phosphor has a particle size in therange from 0.5 μm to 30 μm, both inclusive, and is made of a silicatephosphor having an orthorhombic structure, and a portion of theluminescent layer located at least on the light extraction surface ofthe blue-light-emitting diode has a substantial thickness in the rangefrom 50 μm to 1000 μm, both inclusive.
 2. The light-emittingsemiconductor device of claim 1, wherein the silicate phosphor has aparticle size in the range from 1 μm to 20 μm, both inclusive.
 3. Thelight-emitting semiconductor device of claim 1, wherein the luminescentlayer has a substantial thickness in the range from 100 μm to 700 μm,both inclusive.
 4. The light-emitting semiconductor device of claim 1,wherein the luminescent layer includes a base material made of atranslucent resin and particles of the yellow/yellowish phosphor, andthe particles of the yellow/yellowish phosphor are dispersed throughoutthe base material.
 5. The light-emitting semiconductor device of claim4, wherein the luminescent layer is formed by curing a phosphor pastehaving the yellow/yellowish phosphor and the resin, and the phosphorpaste contains ultra-fine particles including primary particles havingan average particle size in the range from 1 nm to 100 nm, bothinclusive.
 6. The light-emitting semiconductor device of claim 1,wherein the blue-light-emitting diode emits light having a main emissionpeak in the wavelength range greater than 430 nm and less than or equalto 500 nm.
 7. The light-emitting semiconductor device of claim 6,wherein the yellow/yellowish phosphor emits light having a main emissionpeak in the wavelength range from 550 nm to 600 nm, both inclusive. 8.The light-emitting semiconductor device of claim 7, wherein theblue-light-emitting diode is made of a semiconductor selected from thegroup consisting of a gallium nitride-based compound semiconductor, azinc selenide semiconductor and a zinc oxide semiconductor.
 9. Thelight-emitting semiconductor device of claim 8, wherein the color oflight emitted by the light-emitting semiconductor device has an emissionchromatical point (x, y) in the ranges 0.21≦x≦0.48 and 0.19≦y≦0.45,respectively, in a CIE chromaticity diagram.
 10. The light-emittingsemiconductor device of claim 9, wherein blue light emitted by theblue-light-emitting diode passes through the luminescent layer so thatthe color of the fluorescence emitted by the phosphor is added to thecolor of the blue light, thereby producing white light.
 11. Thelight-emitting semiconductor device of claim 10, wherein the luminescentlayer further includes a red/reddish phosphor having a main emissionpeak in the red/reddish wavelength range greater than 600 nm and lessthan or equal to 660 nm.
 12. The light-emitting semiconductor device ofclaim 10, wherein the luminescent layer further includes agreen/greenish phosphor having a main emission peak in thegreen/greenish wavelength range greater than or equal to 500 nm and lessthan 550 nm.
 13. The light-emitting semiconductor device of claim 12,wherein the green/greenish phosphor is a silicate phosphor containing,as a main component, a compound expressed by the chemical formula:(Sr_(1−a3−b3−x)Ba_(a3)Ca_(b3)Eu_(x))₂SiO₄ (where 0≦a3≦1, 0≦b3≦1, 0<x<1).14. The light-emitting semiconductor device of claim 10, wherein theluminescent layer includes a plurality of said silicate phosphors eachof which emits yellow/yellowish light having a main emission peak in thewavelength range from 550 nm to 600 nm, both inclusive and mutuallydiffers in composition.
 15. The light-emitting semiconductor device ofclaim 10, wherein the blue-light-emitting diode is a flip-chip-typeblue-light-emitting diode, the flip-chip-type blue-light-emitting diodeis mounted on a submount element to be electrically connected thereto,and the luminescent layer within which particles of the yellow/yellowishphosphor exist encapsulates the flip-chip-type blue-light-emittingdiode.
 16. The light-emitting semiconductor device of claim 10, whereinthe blue-light-emitting diode is mounted in a cup provided in a mountlead to be electrically connected to the cup, the luminescent layerwithin which particles of the yellow/yellowish phosphor exist isprovided in the cup.
 17. The light-emitting semiconductor device ofclaim 10, wherein the blue-light-emitting diode is placed in a casing,and the luminescent layer within which particles of the yellow/yellowishphosphor exist is provided in the casing.
 18. The light-emittingsemiconductor device of claim 10, wherein the surface of the portion ofthe luminescent layer located at least on the main light-extractingsurface of the blue-light-emitting diode is flat and parallel to themain light-extracting surface.
 19. A light-emitting semiconductor systemcomprising the light-emitting semiconductor device of claim 1.